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UNIVERSITY  OF  CALIFORNIA 
AT   LOS  ANGELES 


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AND    OTHER 

ADVANCED  PRIMERS 

OF  ELECTRICITY. 

BY 

EDWIN  J.  HOUSTON,  A.  M., 

PROFESSOR  OF  NATURAL  PHILOSOPHY  AND   PHYSICAL   GEOGRAPHY  IN 
THE  CENTRAL  HIGH  SCHOOL  OF  PHILADELPHIA  ;  PROFESSOR 
OF  PHYSICS  IN  THE  FRANKLIN  INSTITUTE  OF  PENNSYL- 
VANIA j  ELECTRICIAN  OF  THE  INTERNATIONAL 
ELECTRICAL  EXHIBITION,  ETC. 

AUTHOR   OF 

"  A  DICTIONARY  OF  ELECTRICAL  WORDS,  TERMS  AND  PHRASES,  " 
"  ELEMENTS  OF  PHYSICAL  GEOGRAPHY,"  ETC. 


NEW  YORK  : 

THE  W..J.  JOHNSTON  COMPANY,  LIMITED, 
41  PARK  Row  (TIMES  BUILDING). 


LONDON  : 
WHITTAKEK  &  CO.,  PATERNOSTER  SQUARE. 


COPYRIGHT,  1893,  BY 

THE  W.  J.  JOHNSTON  CO.,  LTD. 


',•8  I 

P  R  E  FA  OE. 


In  presenting  to  the  public  the  volume  entitled 
"  Electrical  Measurements  and  Other  Advanced 
Primers  of  Electricity/'  the  author  again  calls 
attention  to  the  fact  that  these  primers  are  in 
no  sense  to  be  regarded  as  revisions  of  the  "  Primers 
of  Electricity,"  published  in  Philadelphia  in  1884, 
during  the  International  Electrical  Exhibition. 

The  International  Electrical  Exhibition  Primers, 
written  during  the  early  days  of  the  Exhibition,  were 
intended  to  explain  merely  the  elementary  principles 
of  electricity  to  a  public,  that  was  then  almost 
entirely  ignorant  of  even  the  rudiments  of  the  science. 

As  is  well  known,  the  times  have  greatly  changed 
1884.  Multitudinous  commercial  applications 
of  electricity  have  rendered  it  no  longer  optional 
whether  or  no  the  general  public  shall  be  acquainted 
with  the  principles  of  electrical  science.  Such 
knowledge  has  become  a  necessary  part  of  everyday 
business  life. 

Primers,  therefore,  based  on  the  simple  lines  of 
those  of  1884,  would  now  occupy  but  a  compara- 
(3) 


379427 


4  PREFACE. 

tively  limited  field,  and  the  author  has  therefore  en- 
tirely rewritten  his  earlier  primers  and  greatly  en- 
larged their  scope. 

In  these  days  of  voluminous  electrical  literature 
the  student  is  often  in  doubt  as  to  the  best  books 
with  which  to  begin  his  studies.  As  an  aid  in  this 
direction,  and  as  a  species  of  University  Extension 
work,  there  has  been  placed  at  the  end  of  each 
primer  extracts  from  one  or  more  standard  electrical 
books,  so  as  to  give  the  student  some  idea  of  their 
character,  and  thus  enable  him  to  intelligently  select 
the  works  best  suited  to  his  needs. 

The  author  desires  to  acknowledge  his  indebted- 
ness to  Mr.  T.  C.  Martin  and  Mr.  Joseph  Wetzler 
for  critical  revision  of  some  of  the  chapters. 

EDWIN  J.  HOUSTON. 
CENTRAL  HIGH  SCHOOL, 

Philadelphia,  Pa., 

January,  1893. 


CONTENTS. 


I.  The  Measurement  of  Electric  Currents        -  7 

II.  The  Measurement  of  Electromotive  Force  -      39 

III.  The  Measurement  of  Electric  Resistances  -  61 

IV.  Voltaic  Cells             ....          .  -85 
V.  Thermo-Electric    Cells     and   Other    Electric 

Sources     ------  107 

VI.  Distribution  of  Electricity  by  Direct  or  Con- 
tinuous Currents    .....  125 

VII.  Arc  Lighting 145 

VIII.  Incandescent  Electric  Lighting              -  -    163 

IX.  Alternating  Currents  185 

X.  Alternating  Current  Distribution           -  -    207 

XI.  Electric  Currents  of  High  Frequency           -  227 

XII.  Electro- Dynamic  Induction                     -  -    253 

XIII.  Induction  Coils  and  Transformers     -           -  269 

XIV.  Dynamo-Electric  Machines           -           -  -    287 
XV.  Electro-Dynamics           -          -          -          -  311 

XVI.  The  Electric  Motor              -          -          -  -    327 

XVII.  The  Electric  Transmission  of  Power            -  349 

XVIII.  Review— Primer  of  Primers          -  -    367 


(5) 


I.— THE    MEASUREMENT    OF  ELECTRIC 
CURRENTS. 


There  are  various  methods  for  measuring  the 
current  that  passes  in  any  circuit.  These  methods 
are  based  on  the  electrolytic,  the  heating,  or  the  mag- 
netic power  of  the  current.  They  may,  therefore, 
be  arranged  under  the  following  heads  ;  namely: 

(1.)  The  voltametric  method,  or  by  the  use  of  volt- 
ameters, based  on  the  electrolytic  power  of  the  cur- 
rent. 

(2.)  The  calorimetric  method,  or  by  the  use  of 
apparatus  called  calorimeters,  based  on  the  heat  pro- 
duced by  the  current. 

(3.)  The  magnetic  method,  or  by  the  use  of 
various  apparatus  called  galvanometers,  etc.,  based 
on  the  deflections  of  a  readily  movable  magnetic 
needle,  core,  or  movable  circuit  by  the  magnetic  field 
produced  by  the  current. 

(4.)  The  indirect  method,  or  that  in  which   the 
values  of  the  electromotive  force  and  the  resistance 
are  obtained,  and  that  of  the  current  strength  cal- 
culated therefrom  by  the  well  known  formula : 
(7) 


ELECTRICAL  MEASUREMENTS. 


In  the  voltametric  method  an  instrument  called  a 
voltameter  is  employed,  the  operation  of  which  is 
dependent  on  the  fact  that  the  strength  of  current 
flowing  in  any  circuit,  or  more  correctly  the  quantity 
of  electricity  or  the  number  of  coulombs  that 
pass  per  second  through  such  circuit,  can  be  de- 
termined from  the  amount  of  chemical  decomposi- 
tion effected. 

Various  chemicals  are  employed  for  such  purposes  ; 
the  principal  of  these  are  dilute  sulphuric  acid  or 
solutions  of  copper  sulphate  or  of  silver  nitrate. 

It  can  be  shown  that  an  electric  current  equal 
to  one  coulomb  per  second,  or  one  ampere,  will 
deposit  .00111815  gramme,  or  .01725  grain  of  silver 
per  second,  or  will  decompose  .00009326  gramme, 
or  .001439  grain  of  dilute  sulphuric  acid  of  a  specific 
gravity  of  about  1.1  per  second. 

Voltameters  may  be  divided  into  two  general 
classes;  namely,  volume  voltameters  and  weight 
voltameters.  In  the  first  the  quantity  of  electricity 
passing  is  determined  by  the  volume  of  gas  evolved  ; 
in  the  second  by  the  weight  of  material  decomposed 
after  the  current  has  passed  for  a  given  time. 

In  the  sulphuric  acid  voltameter  shown  in  Fig.  1, 


THE  MEASUREMENT  OF  ELECTRIC  CURRENTS.      9 

electrodes  of  platinum  are  immersed  in  sulphuric 
acid  of  the  specific  gravity  of  1.1.  On  the  passage  of 
the  current  hydrogen  appears  at  the  negative  terminal 
and  oxygen  at  the  positive  terminal,  in  the  propor- 
tion of  about  two  volumes  to  one.  The  electro- 
motive force  of  the  battery  must  not  be  less  than 
1.44  volts,  or  electrolysis  will  not  occur. 


FIG.  1.— A  SULPHURIC  ACID  VOLTAMETER. 

The  sulphuric  acid  voltameter  may  be  arranged 
either  as  a  volume  or  as  a  weight  voltameter.  In 
the  form  shown  in  Fig.  1,  it  is  arranged  as  a 
volume  voltameter.  When  arranged  as  a  weight  volt- 
ameter, the  evolved  gas  escapes  through  a  drying 
tube  containing  calcium  chloride.  This  tube  is 
provided  to  stop  the  acid  liquor  that  may  be  me- 
chanically carried  over  with  the  disengaged  gases. 

The  weight  of  sulphuric  acid  decomposed  is  de- 


10  ELECTRICAL  MEASUREMENTS. 

termined  from  the  decrease  in  weight  of  the  instru- 
ment after  the  current  has  been  passed  for  a  given 
time. 

The  form  generally  given  to  the  weight  voltameter 
is  that  in  which  the .  current  is  passed  between 
plates  of  copper  immersed  in  an  electrolyte  of  cop- 
per sulphate,  or  of  silver  immersed  in  silver  nitrate. 
In  either  case  the  amount  of  current  passing  is  de- 
termined by  the  increased  weight  of  one  of  the 


If  several  voltameters  of  different  sizes  and  shapes 
are  placed  in  a  circuit  in  series,  and  the  same  kind 
of  liquid  is  placed  in  each  instrument,  such,  for  ex- 
ample, as  copper  sulphate,  and  a  current  is  passed 
successively  through  them,  it  will  be  found  that  the 
amount  of  deposition  as  determined  by  the  increase 
of  copper  on  one  of  the  plates  is  the  same  in  each  case. 
In  other  words,  the  amount  of  chemical  decomposi- 
tion effected  is  independent  of  the  size,  shape  or  con- 
struction of  the  voltameter,  and  depends  only  on  the 
strength  of  the  current  that  passes.  This  fact  ren- 
ders the  voltameter  a  ready,  though  not  very  reliable, 
means  for  determining  the  strength  of  a  current. 

To  determine  accurately,  however,  the  current 
strength  passing  by  such  means,  a  fairly  consider- 
able time  is  necessary,  and  during  this  time  the  cur- 


THE  MEASUREMENT  OF  ELECTRIC  CURRENTS.    H 

rent  strength  is  exceedingly  apt  to  vary.  For  this 
reason  other  methods  are  generally  adopted  in  prac- 
tice for  the  measurement  of  current  strength. 

The  calorimetric  method  is  based  on  the  fact  that 
the  strength  of  current  passing  in  any  circuit 
may  be  determined  from  the  energy  liberated 
in  such  circuit  as  indicated  by  means  of  the 
increase  in  temperature  produced  in  the  cir- 


Fio.  2.— ELECTRIC  CALORIMETER. 

cuit.  This  increase  in  temperature  is  deter- 
mined by  means  of  an  instrument  called  an  electric 
calorimeter.  It  consists,  as  shown  in  Fig.  2,  of  a 
vessel  provided  with  a  liquid  which  surrounds  a  por- 
tion of  the  circuit  N  M,  immersed  therein.  A  ther- 
mometer, T,  is  provided  for  measuring  the  increase 
in  temperature. 


12  ELECTRICAL  MEASUREMENTS. 

The  indications  of  the  calorimeter  are  based  on 
the  increase  in  temperature  in  a  given  time  of  a 
weighed  quantity  of  water  or  other  liquid  in  the 
calorimeter.  This  increase  is  proportional : 

(1.)  To  the  resistance  of  the  conductor. 

(2.)  To  the  square  of  the  strength  of  the  current 
passing. 

(3.)  To  the  time  the  current  is  passing. 

In  the  galvanometric  method  the  strength  of  the 
current  passing  is  determined  by  the  use  of  certain 
instruments  called  galvanometers.  This-  method  is 
generally  employed  in  commercial  determinations 
of  current  strength,  because  such  strength  can  be 
thus  determined  by  a  single  observation. 

A  galvanometer  is  an  instrument  for  measuring 
the  strength  of  an  electric  current  by  means  of  the 
deflection  of  a  magnetic  needle.  The  galvanometer 
was  invented  by  Schweigger,  and  is  based  on  the  dis- 
covery by  Oersted  of  the  power  which  an  electric 
current  possesses  of  deflecting  a  magnetic  needle 
placed  near  it. 

This  deflection,  as  we  have  already  seen,  is  due  to 
the  mutual  action  which  exists  between  the  field  of 
the  magnet  and  the  field  of  the  current. 

The  following  principles  should  be  borne  in  mind 
in  the  study  of  the  galvanometer  : 


THE  MEASUREMENT  OF  ELECTRIC  CURRENTS,    13 

(1.)  The  direction  of  the  deflection  of  the  mag- 
netic needle  will  depend  both  on  the  direction  in 
which  the  current  flows  in  the  deflecting  circuit  and 
on  the  relative  positions  of  the  circuit  and  the  needle 
to  each  other. 

(2.)  No  matter  in  what  direction  the  needle  is 
deflected  it  will,  if  the  current  is  sufficiently  strong, 
A  B  C 


FIG.  3.— AMPERE'S  APPARATUS. 

always  tend  to  come  to  rest  at  right  angles  to   the 
circuit. 

(3.)  In  the  same  instrument  the  amount  of  de- 
flection increases  with  the  strength  of  the  current 
that  passes,  though  not  necessarily  proportionally. 

The  power  possessed  by  an  electric  current  of  de- 
flecting a  magnetic  needle  can  be  readily  shown  by 
means  of  the  apparatus  represented  in  Pig.  3,  in 


14  ELECTRICAL  MEASUREMENTS. 

which  a  conductor,  D  F  G  E,  bent  in  the  form  of 
a  hollow  rectangle,  is  provided  with  a  readily  mova- 
ble magnetic  needle  M,  supported  at  its  centre. 
Mercury  cups  A,  B  and  C,  serve  to  pass  the  current 
in  different  directions  through  the  apparatus.  In 
experimenting  with  this  apparatus  it  will  be  found 
that  the  direction  in  which  the  needle  is  deflected 
will  depend  on  the  direction  in  which  the  current 
flows. 

Various  methods  have  been  suggested  for  readily 
remembering  the  direction  of  deflection  of  a  mag- 
netic needle  under  various  circumstances  ;  for  exam- 
pie: 

(1.)  The  north  pole  of  a  magnetic  needle  is  de- 
flected to  the  left  hand  of  an  observer  who  is  sup- 
posed to  be  swimming  in  the  current  and  facing  the 
needle. 

(2.)  If  an  ordinary  corkscrew,  placed  along  a 
conductor  through  which  a  current  is  passing,  be 
twisted  so  as  to  advance  or  move  in  the  same  direc- 
tion as  the  current,  the  direction  in  which  its 
handle  must  be  turned  in  order  to  produce  such 
motion  is  the  same  as  the  direction  in  which  the 
needle  will  be  deflected. 

Since  in  all  cases  the  needle  comes  to  rest  with  the 
lines  of  force  of  the  deflecting  field  passing  in  at 


THE  MEASUREMENT  OF  ELECTRIC  CURRENTS.     15 

its  south  pole  and  coming  out  at  its  north  pole,  and 
since  the  lines  of  magnetic  force  of  an  electric  cir- 
cuit form  concentric  circular  lines  around  the  cir- 
cuit, the  needle,  as  can  readily  be  seen,  must  come 
to  rest  with  its  north  pole  pointing  in  diametri- 
cally opposite  directions  on  opposite  sides  of  the  con- 
ductor. 

By  referring  again  to  Fig.  3,  and  remembering  either 
of  the  above  rules  as  to  the  direction  of  the  deflec- 
tion of  the  needle,  it  will  be  seen  that  when  the 
current  passes  through  the  rectangular  circuit  D  E 
G  F,  each  branch  D  E,  E  G,  G  F,  and  F  D,  will 
deflect  the  north  pole  of  the  magnetic  needle  M,  in 
the  same  direction  ;  for,  if  the  current  passes  above 
the  needle  through  the  branch  D  E,  from  left  to 
right,  it  will  pass  through  the  lower  branch,  G  F, 
from  right  to  left,  and  will,  therefore,  deflect  the 
needle  in  the  same  direction.  So  also  the  current 
passes  through  the  vertical  branch  E  G,  in  the  op- 
posite direction  to  that  in  which  it  passes  through 
F  D,  and  these  two  will  also  tend  to  deflect  the 
needle  in  the  same  direction.  Moreover,  all  the 
separate  branches  tend  to  deflect  the  needle  in  the 
same  direction. 

Based  on  this  principle,  Schweigger  constructed 
a  piece  of  apparatus  called  the  multiplier,  in  which 


16  ELECTRICAL  MEASUREMENTS. 

the  deflecting  current  is  caused  to  pass  a  number  of 
times  around  a  coil  of  insulated  wire,  C  C,  Fig.  4, 
wrapped  around  a  hollow  rectangular  frame  as 
shown.  When  a  current  is  passed  through  the  coil 
by  connecting  its  terminals,  N,  P,  to  an  electric 
source,  the  needle  will  be  deflected  to  an  extent  that 
will  increase  with  the  number  of  turns  of  wire  that 
are  placed  in  the  coil  0  C,  provided,  of  course,  the 
current  strength  remains  constant. 


FIG.  4.-SCHWEIGGER'S  MULTIPLIER. 

The  galvanometer  is  based  on  Schweigger's  multi- 
plier. It  consists  of  coils  of  various  shapes  and  sizes 
so  placed,  as  regards  a  readily  movable  magnetic 
needle,  as  to  cause  its  deflection  on  the  passage  of  the 
current.  The  current  strength  is  determined  by  the 
value  of  such  deflections,  and  this  value  will  vary 
according  to  the  size  and  number  of  the  deflecting 


THE  MEASUREMENT  OF  ELECTRIC  CURRENTS.     17 

coils,  the  relative  size  of  the  needle,  and  its  position 
as  regards  the  deflecting  coils. 

In  all  cases,  when  no  current  is  passing,  the 
needle,  when  at  rest,  should  occupy  a  position 
parallel  to  the  plane  of  the  coil.  On  the  passage 
of  the  current  the  needle  tends  to  place  itself  at 
right  angles  to  the  direction  of  the  current,  or  to 
the  plane  of  the  conducting  wire  in  the  coil.  The 


FIG.  5,-AsTATic  GALVANOMETER. 

needle  is  deflected  by  the  current  from  its  position 
of  rest  either  in  the  earth's  field  or  in  the  field 
obtained  from  permanent  or  electro-magnets.  In 
the  first  case,  when  in  use  to  measure  a  current,  the 
plane  of  the  galvanometer  coils  must  coincide  with 
the  plane  of  the  magnetic  meridian  of  the  place. 
In  the  other  case,  the  needle  may  be  used  in  any 
position  in  which  it  is  free  to  move. 


18  ELECTRICAL  MEASUREMENTS. 

Galvanometers  are  constructed  in  a  variety  of 
forms,  either  according  to  the  purpose  for  which 
they  are  employed,  or  the  manner  in  which  their 
deflections  are  valued.  A  very  common  form  given 
to  the  galvanometer  is  shown  in  Fig.  5.  This 
form  is  called  the  astatic  galvanometer  because  it 
employs  an  astatic  needle. 

The  astatic  needle  consists,  as  shown  in  Fig.  6,  of 
two  magnetic  needles,  N  8  and  S'  N1,  placed  verti  • 


FIG.  6.— ASTATIC  NEEDLE. 

cally  one  above  the  other  and  rigidly  connected  to 
the  vertical  axis  a  a,  with  their  opposite  poles,  N  8 
and  S'  N',  opposed  to  each  other. 

The  idea  of  employing  an  astatic  needle  in  this 
form  of  galvanometer  is  to  increase  the  sensitiveness 
of  the  instrument,  and  thus  obtain  a  greater  deflec- 
tion by  the  use  of  a  smaller  current. 

When  an  astatic  needle  is   employed,  the  upper 


THE  MEASUREMENT  OF  ELECTRIC  CURRENTS.     19 

needle  is  placed  outside  the  deflecting  coil,  and  the 
lower  needle  inside  the  coil,  as  shown  in  Fig.  7. 

Since  the  current  passes  under  the  needle  S  N, 
in  the  opposite  direction  to  that  in  which  it  passes 
over  it,  both  of  these  portions  of  the  circuit  deflect  the 
needle  in  the  same  direction.  The  upper  needle, 
8'  N',  is  deflected  by  the  portion  of  the  current 
flowing  below  it  in  the  same  direction  as  the  lower 
needle,  N  8,  because  its  poles  are  oppositely  directed. 


FIG.  7.— ASTATIC  NEEDLE  AND  DEFLECTING  CIRCUIT. 

In  very  sensitive  galvanometers  two  coils  are  em- 
ployed, the  one  surrounding  the  lower  needle  and  the 
other  surrounding  the  upper  needle.  These  coils  are 
so  wound  that  their  deflecting  action  on  both  needles 
is  in  the  same  direction.  In  some  forms  of  galva- 
nometers the  sensitiveness  of  the  instrument  is  varied 
by  means  of  a  magnet  called  a  compensating  magnet, 
placed  on  an  axis  above  the  magnetic  needle.  As 


20  ELECTRICAL  MEASUREMENTS. 

the  compensating  magnet  is  moved  toward  or  from 
the  needle,  the  effects  of  the  earth's  field,  and,  con- 
sequently, the  sensitiveness  of  the  galvanometer,  are 
varied. 

Galvanometers  for  commercial  use  assume  a  vari- 
ety of  forms.  Their  scales  are  generally  so  divided 
as  to  enable  the  amperes,  volts,  ohms,  or  watts,  etc., 
to  be  read  off  directly.  Such  instruments  are  called 
ampere-meters  or  ammeters,  voltmeters,  ohm-meters, 
watt-meters,  etc. 

By  the  sensibility  of  a  galvanometer  is  meant  the 
readiness  with  which  its  needle  is  deflected  by  the 
passage  of  a  current,  as  well  as  the  extent  of  such 
deflection.  The  reciprocal  of  the  current  required 
to  produce  a  deflection  of  one  degree  is  called  the 
figure  of  merit  of  the  galvanometer.  The  smaller 
the  current  required  to  produce  this  deflection  the 
greater  is  the  figure  of  merit,  and,  consequently, 
the  greater  is  the  sensitiveness  of  the  galvanometer. 

In  order  to  increase  the  effective  length  of  the 
needle  without  increasing  its  actual  length,  the  ex- 
pedient is  sometimes  ad  opted  of  reading  the  deflections 
of  the  needle,  not  by  the  motion  of  the  needle  itself, 
but  by  the  motion  of  a  spot  of  light  reflected  from  a 
mirror  attached  to  the  suspension  system  of  the  needle, 
so  as  to  move  the  spot  of  light  over  a  distant  scale. 


THE  MEASUREMENT  OF  ELECTRIC  CURRENTS.    31 

A  well  known  form  of  mirror  galvanometer,  de- 
vised by  Sir  William  Thomson,  is  shown  in  Fig.  8. 
The  needle  is  attached  to  the  back  of  a  light  silvered 
concave  mirror,  suspended  by  means  of  a  single  silk 
fibre,  and  is  placed  inside  a  coil  of  insulated  wire. 
In  some  mirror  galvanometers  the  mirror  is  attached 
directly  to  the  suspension  fibre. 


FIG.  8.— MIRROR  GALVANOMETER. 

The  compensating  magnet  N  S,  is  used  to  vary 
the  sensitiveness  of  the  instrument. 

The  lamp  L,  is  placed  at  the  back  of  a  slot  in  a  white 
screen,  on  the  other  face  of  which  is  placed  a  gradu- 
ated scale  K.  The  light  which  passes  through  this 
slot  is  reflected  from  the  mirror  as  a  bright  spot  of 
light  which  is  caused  to  fall  on  the  graduated  scale. 

In  Fig.  9  the  details  of  the  slot  and  back  of 
screen  are  shown. 


22  ELECTRICAL  MEASUREMENTS. 

A  form  of  galvanometer  called  the  sine  galvanom- 
eter is  shown  in  Fig..  10. 

This  galvanometer  differs  from  other  galva- 
nometers in  the  fact  that  its  vertical  coil,  M,  is  mov- 
able about  a  vertical  axis,  so  that  the  coil  can  readily 
be  made  to  follow  the  needle  in  its  deflections,  and 
so  be  kept  parallel  to  it.  The  needle  is  placed  in- 
side the  coil,  and  can  be  of  any  length  smaller  than 


KIG.  9  —DETAILS  OF  GALVANOMETER  LAMP  AND  SCALE. 


the  diameter  of  the  coil.     Its  deflections  are  marked 
on  the  horizontal  graduated  scale  at  N. 

When  used  to  measure  a  current,  the  vertical  coil 
M,  is  moved  on  a  vertical  axis  over  the  horizontal 
graduated  circle  H,  so  as  to  keep  the  coil  parallel 
with  the  needle.  This  vertical  coil  M,  is  moved  until 
the  needle  shows  no  further  deflection,  though  the 
coil  is  parallel  with  the  axis  of  the  needle.  The 


THE  MEASUREMENT  OF  ELECTRIC  CURRENTS     33 

strength  of  the  current  is  then  inferred  from  the 
amount  of  movement  which  has  been  given  to  the 
vertical  coil  over  the  horizontal  circle  H ;  or,  what 
is  the  same  thing,  by  the  distance  the  needle  has 


FIG.  IO.-SINE  GALVANOMETER. 


been  deflected  from  its  original  position  of  rest  in 
the  earth's  field. 

In  the  sine  galvanometer  the  current  strength  is  pro- 
portional to  the  sine  of  the  angle  through  which  the 


24  ELECTRICAL  MEASUREMENTS. 

magnetic  needle  is  deflected.  This  angle  is  most 
conveniently  measured  on  the  horizontal  graduated 
circle  H,  as  the  angle  through  which  it  is  necessary 
to  move  the  coil  M,  from  its  position  when  the  needle 
is  at  rest  in  the  plane  of  the  earth's  meridian,  to  the 
position  in  which  the  needle  is  no  longer  deflected 
by  the  current  passing  through  its  coils,  although 
they  are  still  parallel  to  the  needle. 


FIG.  ll.— TANGENT  GALVANOMETER. 

In  the  tangent  galvanometer  the  strength  of  cur- 
rent passing  through  the  galvanometer  coils  is  pro- 
portional to  the  tangent  of  the  angle  through  which 
the  needle  is  deflected.  In  this  form  of  galva- 
nometer the  coil  is  fixed  and  the  strength  of  the 
current  passing  is  proportional  to  the  tangent  of 
the  angle  of  deflection,  provided  the  length  of  the 


THE  MEASUREMENT  OF  ELECTRIC  CURRENTS.    25 

magnetic  needle  is  less  than  one-twelfth  the  diameter 
of  the  coil. 

A  form  of  tangent  galvanometer  is  shown  in  Fig. 
11.  Tangent  galvanometers,  when  used  to  measure 
large  currents,  consist  often  of  but  a  single  turn  of 
wire.  They  are,  however,  frequently  formed  of  a 
number  of  turns  like  other  galvanometers.  Great 
care  must  be  taken  to  suspend  the  needle  at  the  ex- 
act centre  of  the  deflecting  coil,  and  not  to  permit 
its  length  to  exceed  the  above  mentioned  propor- 
tions. 

It  is  often  necessary  to  make  galvanometric 
measurements  on  shipboard.  Instruments  for  such 
purposes  are  generally  termed  marine  galvanometers. 
In  order  to  avoid  the  disturbing  action  caused  by  the 
rolling  of  the  ship,  the  magnetic  needle  is  suspended 
by  a  silk  fibre  attached  above  and  below  in  a 
vertical  line  with  the  centre  of  gravity  of  the  needle. 
It  is  also  necessary  to  protect  the  needle  from  the  in- 
fluence of  magnetized  masses  of  iron  in  motion.  To 
effect  this  the  needle  of  such  galvanometer  is  shielded 
from  extraneous  magnetic  fields  by  means  of  a 
magnetic  screen,  which  consists  essentially  of  an  iron 
box  within  which  the  whole  galvanometer  is  placed. 
In  the  differential  galvanometer  the  needle  is  de- 
flected by  two  coils  of  wire  that  are  so  wound  and 


26  ELECTRICAL  MEASUREMENTS, 

placed  as  to  produce  deflections  in  opposite  direc- 
tions. The  needle  is,  therefore,,  deflected  to  an 
amount  equal  to  the  difference  of  the  deflecting 
forces. 


FIG.  12. -DIFFERENTIAL  GALVANOMETER. 

When  currents  of  equal  strength  flow  through 
each  of  the  coils  no  deflection  of  the  needle  takes 
place,  since  each  coil  neutralizes  the  other's  effects, 
so  far  as  its  action  on  the  needle  is  concerned. 

One  form  of  differential  galvanometer  is  shown  in 


THE  MEASUREMENT  OF  ELECTRIC  CURRENTS.     21 

Fig.  13.  In  some  cases  the  separate  coils  may  be 
so  connected  that  each  tends  to  deflect  the  needle 
in  the  same  direction.  In  such  cases,  of  course,  the 
galvanometer  ceases  to  be  differential  in  action. 

The  form  of  galvanometer  shown  in  Fig.  13  is 
called,  after  its  inventors,  the  Deprez-D'Arsonval 
galvanometer.  This  form  of  galvanometer  is  also 


FlO.  13.-DEPREZ-D'ARSONVAL  GALVANOMETER. 

called  a  dead-beat  galvanometer,  from  the  fact  that 
its  needle  moves  rapidly  over  the  scale  to  its  position, 
and  comes  to  rest  without  moving  alternately  on 
either  side  of  its  position  of  rest  for  a  number  of 
times,  as  is  common  in  most  forms  of  instruments. 

The  movable  part  of  the  Deprez-D'Arsonval  gal- 
vanometer consists  of  a  light  rectangular  coil    C, 


ELECTRICAL  MEASUREMENTS. 


formed  of  many  turns  of  wire  supported  by  two 
single  wires,  II J  and  D  E,  between  the  poles  of  a 
strong,  permanent  horseshoe  magnet  A  A.  The 
dead-beat  action  is  due  to  the  fact  that  the  motions 
of  the  coil  under  the  action  of  the  deflecting  current 


1 


FIG.  14.— TORSION  GALVANOMETER. 

produce  a  current  that  tends  to  oppose  such  motion. 
The  movements  of  the  coil  are  observed  by  means  of 
a  spot  of  light  reflected  from  the  mirror  J,  fixed  to 
the  wire  HJ. 


THE  MEASUREMENT  OF  ELECTRIC  CURRENTS.    29 

111  the  torsion  galvanometer  the  strength  of  the 
deflecting  current  is  measured  by  the  torsion  it  ex- 
erts on  the  suspension  system. 

In  one  form  of  torsion  galvanometer  the  magnetic 
needle  consists  of  a  bell-shaped  magnet,  suspended 
by  a  thread  and  spiral  spring  between  two  deflect- 
ing coils  of  insulated  wire,  placed  on  either  side  of 
a  magnet,  as  shown  in  Fig.  14. 


FIG.  15.— VERTICAL  GALVANOMETER. 

The  strength  of  the  current  to  be  measured  is  de- 
termined by  the  amount  of  torsion  required  to  bring 
the  magnetic  needle  back  to  its  zero  position,  while 
under  the  deflecting  power  of  the  current.  A 
horizontal  scale  is  placed  on  the  top  of  the  instru- 
ment for  accurately  measuring  the  angle  of  torsion. 

In  the  case  of  currents  which  continue  for  but  a 
moment  in  one  direction,  for  example,  as  that  pro- 


80  ELECTRICAL  MEASUREMENTS. 

duced  by  the  discharge  of  a  condenser,  a  form  of 
galvanometer  known  as  the  ballistic  galvanometer  is 
employed. 

Galvanometers  are  sometimes  divided  into  horizon- 
tal and  vertical  galvanometers,  according  to  the  po- 
sition in  which  their  needles  are  free  to  move.  A 
vertical  galvanometer  is  shown  in  Fig.  15.  In  such 


FIG.  16.— DETECTOR  GALVANOMETER. 

an  instrument  the  north  pole  of  the  needle  is  weight- 
ed, so  that  when  no  current  is  passing  through  the 
coils  the  needle  points  vertically  downward  to  the 
zero  on  the  scale. 

Most  galvanometers  are  provided  with  horizontal 
needles.  In  the  form  shown  in  Fig.  16,  which  is 
called  a  detector  galvanometer,  the  needle  is  hori- 
zontal. Such  an  instrument  is  suitable  for  readily 
detecting  the  presence  of  a  current  in  any  circuit. 


THE  MEASUREMENT  OF  ELECTRIC  CURRENTS.     31 

The  form  of  galvanometer  shown  in  Fig.  17  is 
known  as  Siemens'  electro-dynamometer,  and  is 
suitable  for  the  measurement  of  commercial  cur- 
rents. 


FIG.  17.— SIEMENS'  ELECTRO-DYNAMOMETER. 
The  electro-dynamometer  contains  two  coils,  the 
fixed  coil  C,  secured  as  shown  to  the  upright  support, 
and  the  movable  coil  L,  consisting  of  but  a  single 


32  ELECTRICAL  MEASUREMENTS. 

turn  of  wire.  The  movable  coil  is  suspended  by 
means  of  a  thread  and  a  delicate  spring  S,  capable 
of  being  twisted  through  an  angle  of  torsion  that  is 
measured  on  the  horizontal  scale  shown  at  the  top  of 
the  figure. 

The  ends  of  the  movable  coil  dip  into  mercury 
cups,  so  placed  in  the  circuit  that  the  current  to  be 
measured  passes  in  series  through  the  fixed  and 
the  movable  coils. 

When  ready  for  use  the  movable  coil  is  placed  at 
right  angles  to  the  fixed  coil.  On  the  passage  of  the 
current  to  be  measured,  the  mutual  action  which 
these  coils  exert  on  each  other  tends  to  place  the 
movable  coil  parallel  to  the  fixed  coil,  against  the 
torsion  of  the  spring  S.  The  amount  of  the  deflect- 
ing force  is  determined  by  the  amount  of  torsion 
required  to  bring  the  movable  coil  back  to  its  zero 
position. 

In  Siemens' electro-dynamometer,  unlike  the  tor- 
sion galvanometer  already  described,  since  the  action 
of  the  fixed  and  movable  coils  is  mutual,  the  cur- 
rent is  proportional  to  the  square  root  of  the 
angle  of  torsion,  and  not,  as  in  the  torsion  galva- 
nometer, to  the  angle  of  torsion.  Or,  in  other  words, 
in  the  electro-dynamometer  the  deflecting  force  is 
proportional  to  the  square  of  the  deflecting  current 


THE  MEASUREMENT  OF  ELECTRIC  CURRENTS.     33 

strength,  while  in  the  torsion  galvanometer  it  is 
merely  proportional  to  the  current  strength. 

In  the  commercial  distribution  of  electricity  for 
the  various  purposes  of  light  and  power  some  form 
of  instrument  is  required  for  measuring  and  record- 
ing the  current  which  is  supplied  to  each  consumer  ; 
or,  more  correctly,  the  quantity  of  electricity  that 
passes  in  a  given  time  through  any  consumption  cir- 
cuit. The  purpose  of  such  apparatus  is  the  same  as 
that  of  the  meters  employed  to  determine  the  quan- 
tity of  illuminating  gas  consumed.  They  are  for 
this  reason  generally  called  electric  meters. 

Electric  meters  are  constructed  in  a  great  variety 
of  forms;  these,  however,  may  be  divided  as  follows, 
namely : 

(1.)  Electro-magnetic  meters,  or  those  in  which 
the  current  passing  is  measured  by  the  magnetic 
effects  it  produces. 

In  electro-magnetic  meters  the  entire  current  may 
be  passed  through  the  meter. 

(2.)  Electro-chemical  meters,  or  those  in  which  the 
current  passing  is  measured  by  the  electrolytic  de- 
composition which  it  effects. 

In  electro-chemical  meters  a  known  fractional  or 
shunted  portion  of  the  current  is  passed  through  a 
solution  of  a  metallic  salt,  and  the  current  strength 


34  ELECTRICAL  MEASUREMENTS. 

is  determined  by  the  amount  of  electrolytic  decom- 
position effected. 

(3.)  Electro-thermal  meters,  or  those  in  which  the 
current  passing  is  measured  by  movements  effected 
by  the  increase  of  temperature  of  a  resistance 
through  which  the  current  passes,  or  by  the  differ- 
ence of  weight  produced  by  the  evaporation  of  a 
liquid  by  means  of  heat  generated  by  the  current. 

(•i.)  Electric  time  meters,  or  those  in  which  no 
attempt  is  made  to  measure  the  current  passing, 
but  in  which  a  record  is  kept  of  the  number  of  hours 
that  the  current  passes  through  the  consumption 
circuit. 

Edison's  electric  meter  is  of  the  second  class.  It 
consists  of  two  voltameters  formed  of  plates  of  zinc 
dipped  in  a  solution  of  zinc  sulphate.  These  plates 
are  weighed  at  stated  intervals — one  every  month, 
and  the  other  every  three  months. 


THE  MEASUREMENT  OF  ELECTRIC  CURRENTS.    35 


EXTRACTS  FROM  STANDARD  WORKS. 

Concerning  the  relative  advantages  of  -voltameters 
and  galvanometers  in  the  measurement  of  electrical 
currents,  Ayrton,  in  his  "  Practical  Electricity,"* 
speaks  thus  on  page  20  : 

The  disadvantage  of  employing  a  voltameter  for  the  prac- 
tical measurement  of  currents  is  that  it  requires  a  strong 
current  to  produce  any  visible  decomposition  in  a  reasona- 
ble time.  Even  the  current  of  one  ampere,  which  is  about 
that  used  in  an  ordinary  Swan  incandescent  lamp,  would 
require  two  hours  fifty-eight  minutes  and  forty  five  sec- 
onds to  decompose  one  gramme  of  dilute  sulphuric  acid, 
whereas  the  weak  currents  used  in  telegraphy,  and,  still 
more,  the  far  weaker  currents  used  in  testing  the  insulating 
character  of  specimens  of  gutta-percha,  india-rubber,  etc., 
might  pass  for  many  days  through  a  sulphuric  acid  volta- 
meter before  their  presence  could  be  detected,  much  less 
their  strength  measured.  Indeed,  not  to  mention  the 
enormous  waste  of  time,  and  the  difficulty  of  keeping  the 
current  strength  which  it  was  desired  to  measure  constant 
all  this  time,  the  leakage  of  gas  which  would  take  place  at 
all  parts  of  the  apparatus  that  were  not  hermetically  sealed 

'"Practical  Electricity :  A  Laboratory  and  Lecture  Course  for 
First  Year  Students  of  Electrical  Engineering,  by  W.  E.  Ayrton, 
F.R.S.  London:  Cassell  &  Co.,  Ld.  1888.  516  pages,  173  illustra- 
tions. Price  $2. 50. 


86  ELECTRICAL  MEASUREMENTS, 

would  render  such  a  mode  of  testing  quite  futile.  Hence, 
although  the  voltametric  method  is  the  most  direct  way  of 
measuring  a  current  strength,  and  although  it  is  constantly 
made  use  of  for  measuring  the  large  currents  now  used  in- 
dustrially, still  the  very  fact  that  the  amount  of  chemical 
decomposition  produced  in  a  given  time  by  a  certain  current 
is  independent  of  the  shape  or  size  of  the  instrument  makes 
it  impossible  to  increase  its  sensibility.  Consequently  some 
other  apparatus  must  be  employed  for  practically  measuring 
small  currents,  and  the  law  of  the  apparatus,  that  is,  the 
connection  between  the  real  strength  of  the  current  and 
the  effect  produced  in  the  apparatus,  must  be  experiment- 
ally ascertained  by  direct  comparison  with  a  voltameter. 

But  if  we  are  going  to  compare  together  the  indications 
of  the  two  instruments  produced  by  various  currents,  the 
second  instrument  cannot  be  much  more  sensitive  than  the 
voltameter,  and  what  advantage  can  arise  from  employing 
such  an  instrument?  This  leads  us  to  the  fact  that,  whereas 
in  a  voltameter  there  is  only  one  way  by  which  the  produc- 
tion of  the  gas  can  be  more  easily  measured,  namely,  by  di- 
minishing the  bore  of  the  graduated  tube  t  (Fig.  5),  up  which 
the  liquid  is  forced  by  the  production  of  the  gas,  there  are 
two  quite  distinct  ways  in  which  the  magnitude  of  the  de- 
flection of  a  "galvanometer"  needle  can  be  more  easily  read. 
The  first  consists  in  using  a  microscope  or  some  magnifying 
arrangement,  or  in  simply  lengthen  ing  the  pointer,  both  of 
which  methods  correspond  with  using  a  tube  of  smaller  bore 
in  a  voltameter;  the  second  consists  in  winding  a  long  fine 
wire,  instead  of  a  shorter  thicker  wire,  on  the  bobbin  of 
the  galvanometer,  and  which  causes  the  deflection  of  the 


THE  MEASUREMENT  OF  ELECTRIC  CURRENTS.    37 

magnet  to  be  greater  with  the  same  current.  This  second 
mode  has  no  analogy  with  any  possible  change  in  a  single 
voltameter. 

Now  experiment  shows  that  a  galvanometer  of  a  par- 
ticular shape  and  size,  and  with  a  definite  magnetic  needle, 
acted  onby  a  definite  controlling  force,  produced,  say,  by  the 
earth's  magnet  ism,  or  by  some  fixed  permanent  magnet,  has 
a  perfectly  definite  law  connecting  the  magnitude  of  the  de- 
flection with  the  strength  of  the  current  producing  it,  al- 
though the  absolute  value  of  the  current  in  amperes  neces- 
sary to  produce  any  particular  deflection  can  be  increased 
or  diminished  by  using  fewer  turns  of  thick  wire  or  more 
turns  of  fine  wire  to  make  a  coil  of  the  same  dimension. 

Bottone,  in  his  book,  "  Electrical  Instrument 
Making  for  Amateurs/'*  thus  describes,  on  page  134, 
the  making  and  mounting  of  a  needle  for  a  tangent 
galvanometer: 

The  tangent  galvanometer  presents  no  difficulty  in  con- 
struction, A  small  lozenge -shaped  "  needle  "  is  made  from 
a  thin  piece  of  watch  spring,  abDut  1  in.  long  and  £  in. 
wide.  This  is"1'  let  down,"  or  softened,  by  being  held  over 
the  flame  of  a  spirit  lamp  until  of  a  dull  red,  and  allowed 
to  cool  gradually.  When  quite  cold  a  small  hole  \  in.  in 
diameter  is  drilled  through  the  centre.  The  "needle"  is 
then  straightened  out,  and  tested  for  centrality ;  and,  if 

*"  Electrical  Instrument  Making  for  Amateurs  :  A  Practical 
Handbook,"  by  S.  H.  Bottone.  Fourth  edition.  London:  Whittaker 
&  Co.  1889.  202  pages,  59  illustrations.  Price  50  cents. 


38  ELECTRICAL  MEASUREMENTS. 

defective,  filed  until  the  hole  corresponds  with  the  centre 
of  gravity.  It  is  then  hardened  by  being  made  red  hot 
over  the  flam.3  of  a  spirit  lamp,  and  being  dropped 
into  cold  water.  It  must  then  be  carefully 
magnetized  by  being  rubbed  at  each  extremity 
with  the  opposite  poles  of  a  good  horseshoe  magnet. 
When  fully  magnetized  it  nvast  be  fitted  with  a  small 
glass  pivot,  made  as  described  at  paragraph  six,  small 
enough  to  enter  the  Tlff  in.  hole  in  the  needle,  and  about  £ 
in  in  length.  Great  care  must  be  exercised  in  the  choice 
of  a  pivot,  which  must  bs  very  perfectly  shaped,  so  as  to 
allow  great  freedom  of  notion  in  the  poised  needle. 
This  point  baing  settled,  the  pivot  is  attached  to  the 
needle  by  means  of  a  m3r.3  trace  of  good  glue,  applied 
to  the  hole  in  the  needle  orly.  The  needle  must  now  be 
poised  by  its  pivot  oa  a  fi  le  steel  sewing  needle  (No.  10 
will  do),  and  any  want  of  perfect  horizontality  must  be 
remedied  while  the  glue  is  still  moist.  When  the  above 
is  quite  dry,  a  very  fine  straw,  about  2£  in.  long,  has 
a  small  hole  made  in  its  centre  (half  way  between  its  two 
extremities)  with  a  rather  coarse  pin ;  then  the  head  of 
the  pivot  is  pushed  through  this  hole  in  the  straw,  so 
as  to  cause  the  straw  to  lie  exactly  at  right  angles  over  the 
needle.  The  merest  trace  of  glue  will  now  cause  the  straw 
to  adhere  to  and  retain  its  position  on  the  glass  pivot.  This 
can  now  be  set  aside  to  dry. 


II.— THE  MEASUREMENT  OF   ELECTRO- 
MOTIVE FORCE. 


.The  electromotive  force  of  a  source,  or  the  differ- 
ence of  potential  between  any  two  points  in  a  cir- 
cuit, can  be  measured  in  various  ways  by  the  use  of 
instruments  called 'voltmeters.  Among  the  most 
important  of  these  methods  are  the  following  : 

(1.)  By  the  use  of  galvanometers,  or  by  galva- 
nometer voltmeters. 

(2.)  By  the  use  of  electrometers,  or  by  electrom- 
eter voltmeters. 

(3.)  By  the  method  of  weighing,  or  by  balance- 
voltmeters. 

In  the  galvanometric  method,  differences  of  po- 
tential are  determined  by  the  quantity  of  electricity 
that  flows  per  second  through  a  given  resistance,  just 
as  the  pressure  of  water  at  any  opening  in  a  vessel 
can  be  determined  by  the  quantity  of  water  that 
flows  out  from  such  opening  per  second.  Differences 
of  potential,  therefore,  may  be  measured  by  means 
of  any  galvanometer  which  measures  the  current  in 
amperes,  provided  the  resistance  of  the  circuit  is 

known.     Galvanometers  constructed  so  as  to  meas- 
(39) 


40  ELECTRICAL  MEASUREMENTS. 

lire  differences  of  potential  are  called  voltmeters,  or, 
more  correctly,  galvanometer-voltmeters. 

In  the  electrometer-voltmeter  the  difference  of 
potential  may  be  employed  to  charge  insulated 
conductors,  and  the  value  of  such  differences  of  po- 
tential determined  from  the  electrostatic  attractions 
and  repulsions  acting  on  a  readily  movable  needle 
suitably  suspended  near  such  charged  conductors. 
This  latter  form  of  voltmeter  is  generally  termed  an 
electrometer,  or  an  electrometer-voltmeter. 

This  method  consists  in  ascertaining  the  force  or 
weight  required  to  overcome  the  attraction  between 
two  oppositely  charged  plates,  or  two  oppositely  ener- 
gized coils,  or  by  measuring  the  repulsion  between 
two  similarly  energized  coils. 

When  a  galvanometer  is  used  as  an  ampere-meter, 
for  determining  the  strength  of  the  current  passing, 
it  is  placed  directly  in  the  main  circuit.  When 
used  as  a  voltmeter,  for  determining  the  difference 
of  potential  between  any  points  in  the  circuit,  it  is 
placed  in  a  shunt  circuit  to  these  points.  The  coils 
of  voltmeters  are  generally  made  of  much  higher 
resistance  than  those  of  ampere-meters. 

According  to  Ohm's  law  : 

E 

C=  — 
R 


MEASUREMENT  OF  ELECTROMOTIVE  FORCE.      41 

Therefore  :  E  =  C  R  ; 

or,  in  other  words,  the  electromotive  force,  or  differ- 
ence of  potential  in  volts,  is  equal  to  the  current  in 
amperes  multiplied  by  the  resistance  of  the  circuit 
in  ohms. 

Galvanometer-Voltmeters  may  be  constructed  in  a 
great  variety  of  forms.  In  all  such  forms  the  differ- 
ence of  potential  is  determined  from  the  deflection 
of  a  magnetic  needle  by  the  magnetic  field  produced 
by  a  current  which  flows  through  a  coil  of  insulated 
wire.  Since  the  resistance  of  the  voltmeter  is  con- 
stant, the  current  passing,  and  hence  the  deflection 
of  the  needle,  will  vary  only  with  the  electromotive 
force. 

In  galvanometer-voltmeters  the  magnetic  field 
produced  by  the  current  may  deflect  the  needle  of 
the  galvanometer — 

(1.)  Against  the  earth's  field. 

(2.)  Against  the  field  of  a  permanent  magnet  or 
an  electro-magnet. 

(3.)  Against  the  action  of  a  spring. 

(4.)  Against  the  force  of  gravity  acting  on  a  weight. 

Instead  of  determining  the  difference  of  potential 
by  varying  the  magnetic  field  of  the  current  pro- 
duced, such  current  may  be  used  to  heat  a  wire,  and 
the  value  of  the  current  strength,  and,  consequently, 


43  ELECTRICAL  MEASUREMENTS. 

the  difference  of  potential,  determined  by  the  move- 
ment of  a  needle  caused  by  the  expansion  of  a  wire. 

There  are,  therefore,  various  forms  which  may  be 
given  to  voltmeters,  and  various  principles  by  which 
they  operate. 

A  form  of  galvanometer-voltmeter  devised  by  Sir 
William  Thomson  is  shown  in  Fig.  17.  A  coil  of 
insulated  wire  A,  whose  resistance  is  over  5,000 
ohms,  acts  on  a  needle  formed  of  a  number  of  short 


FIG.  18.— GALVANOMETER-VOLTMETER. 

parallel  needles  placed  one  above  the  other.  The 
compound  needle  so  formed  has  attached  to  it  a 
light  aluminium  index  moving  over  a  graduated 
scale. 

A  small  circular  magnet  B,  called  the  restoring 
magnet,  placed  over  the  needle,  is  used  to  vary  the 
strength  of  the  earth's  field  at  any  place,  either 
by  moving  the  magnet  itself,  or  by  moving 
the  box  containing  the  compound  magnetic 


MEASUREMENT  OF  ELECTROMOTIVE  FORCE.     43 

needle  toward  or  from  the  deflecting  coil.  The 
indications  of  this  instrument  are  based  on  the  fact 
that  when  a  galvanometer  of  sufficiently  high  resist- 
ance is  connected  with  any  two  points  in  a  circuit, 
the  current  which  passes  through  it,  and,  conse- 
quently, the  deflection  of  its  needle,  is  directly  pro- 
portional to  the  difference  of  potential  between  two 
such  points. 

It  is  not  necessary,  in  measuring  the  difference  of 
potential  between  any  two  points  in  a  circuit  or  con- 
ductor, that  such  differences  of  potential  be  utilized 
to  cause  an  electric  current ;  they  may,  instead,  be 
used  to  produce  charges  in  an  insulated  conductor, 
and  the  differences  of  potential  can  then  be  inferred 
from  the  movements  of  a  needle  as  the  result  of 
electrostatic  attractions  and  repulsions  produced  by 
such  charges  ;  or  they  may  be  determined  from  the 
weight  required  to  balance  such  movements  so  as  to 
prevent  them  from  occurring. 

Some  forms  of  commercial  galvanometers  are  so 
arranged  as  to  measure  directly  the  product  of  the 
current  and  the  difference  of  potential.  Such 
instruments  give  the  watts  in  the  circuit  and  are, 
therefore,  called  wattmeters. 

A  form  of  wattmeter  consists  essentially  of  a  thick 
wire  coil  placed  in  series  in  the  circuit  whose  elec- 


44  ELECTRICAL  MEASUREMENTS. 

trie  power  is  to  be  measured  and  a  thin  wire  coil 
placed  as  a  shunt  around  the  circuit.  These  two 
coils,  instead  of  acting  on  a  needle,  acton  each  other, 
and  the  amount  of  their  deflection  will  be  propor- 
tional to  the  number  of  watts  in  the  circuit. 


FIG.  19.— QUADRANT  ELECTROMETER 


In  the  electrometer-voltmeter  the  difference  of 
potential  is  measured  by  electrostatic  attractions  and 
repulsions.  A  well-known  form  of  such  instrument 


MEASUREMENT  OF  ELECTROMOTIVE  FORCE.     45 

is  seen  in  the  quadrant  electrometer.  In  the  quad- 
rant electrometer  the  differences  of  potential  are 
measured  by  the  attractive  and  repulsive  forces  ex- 
erted by  four  plates  or  quadrants  on  a  light,  charged, 
needle  of  aluminium  suspended  within  them. 

A  form  of  quadrant  electrometer  is  shown  in  Fig. 
19.  The  sectors  of  the  quadrant  are  made  of  brass 
or  other  conducting  metal  shaped  so  as  to  form  a 
hollow  box.  When  placed  together  the  four 


FIG.  20.— QUADRANT  ELECTROMBTER. 

sectors  are  insulated  from  one  another,  but  the 
opposite  pairs  are  connected  by  a  conducting 
wire,  as  shown  in  Fig.  20.  A  light  aluminium 
needle  u,  is  maintained  at  a  constant  electric 
charge  by  being  connected  with  one  of  the  coat- 
ings of  a  Leyden  jar,  or  with  one  of  the  terminals  of  a 
sufficiently  powerful  voltaic  battery.  The  electrom- 
eter needle  is  generally  suspended  by  two  parallel 
silk  fibres  so  as  to  s\v  ing  freely  inside  the  hollow  box 


46  ELECTRICAL  MEASUREMENTS. 

quadrant.  When  at  rest  the  needle  has  its  greatest 
length  exactly  in  the  direction  of  the  slot  or  space 
between  the  two  opposite  pairs  of  sectors,  as  shown 
in  Fig.  20  by  the  dotted  lines. 

When,  now,  the  two  points  of  any  circuit  whose 
difference  of  potential  is  to  be  measured  are  con- 


Fia.  21.— QUADRANT  ELECTROMETER,  SHOWING  SUSPENDED 
NEEDLE. 

nected  to  opposite  pairs  of  quadrants,  the  charge  so 
produced  deflects  the  needle,  and  from  the  amount 
of  this  deflection  the  difference  of  potential  between 
these  points  can  be  calculated.  In  the  forms  of 
electrometers  shown  in  Figs.  19  and  21  the 


MEASUREMENT  OF  ELECTROMOTIVE  FORCE.      47 

amount  of  this  motion  is  determined  by  means  of  a 
spot  of  light  reflected  from  a  mirror  E,  supported  by 
the  suspension  fibre,  and  generally  observed  through 
a  telescope. 

Electrometer-voltmeters  are  better  adapted  than 
galvanometer-voltmeters  for  determining  differences 
of  potential  in  certain  cases  because  they  do  not 
require  the  passage  of  a  current. 


FIG.  22.— ATTRACTED  Disc  ELECTROMETER. 

In  the  form  of  electrometer-voltmeter  shown  in 
Fig.  22,  the  difference  of  potential  is  measured  by 
the  weight  required  to  balance  the  attraction  which 
exists  between  two  oppositely  charged  metallic  discs. 
In  the  form  here  shown,  the  plate  C,  is  suspended 
from  the  longer  end  of  the  lever  I,  within  the  fixed 
guard-plate  or  ring  B,  immediately  above  a  second 
plate  A,  supported  on  an  insulated  stand.  The  plate 


48  ELECTRICAL  MEASUREMENTS. 

A,  can  be  moved  from  or  toward  the  plate  B,  through 
a  measurable  distance. 

The  electrostatic  attraction  is  measured  by  the 
attraction  of  the  fixed  disc  A,  on  the  movable  disc 
C.  These  two  bodies  are  connected  respectively  to 
the  two  points  whose  difference  of  potential  is  to  be 
determined.  One  of  these  may  be  the  earth. 

Instead  of  measuring  the  difference  of  potential 
directly  by  balancing  the  tendency  to  motion  pro- 
S 


Fio.  23.— POTENTIOMETER. 

duced  by  the  attraction  or  repulsion  by  means  of 
a  weight,  such  differences  may  be  determined  by  bal- 
ancing or  opposing  an  unknown  difference  of  poten- 
tial by  a  known  difference  of  potential. 

The  apparatus  shown  in  Fig.  23,  called  the  poten- 
tiometer, is  an  apparatus  of  this  character.  The 
unknown  difference  of  potential  to  be  measured  is 
balanced  or  opposed  by  a  known  difference  of  poten- 
tial, the  equality  of  the  balancing  being  determined 
by  the  failure  of  one  or  more  galvanometers,  placed 


MEASUREMENT  OF  ELECTROMOTIVE  FORCE.     49 

In  the  shunt  circuits,  to  show  any  movements  of 
their  needles. 

A  secondary  battery  S,  or  a  standard  voltaic  cell, 
has  its  terminals  connected  to  the  ends  A  and  B,  of 
a  wire  of  uniform  diameter  and  of  high  resistance 
called  the  potentiometer  wire.  There  will,  therefore, 
occur  along  the  wire  a  fall  or  urop  of  potential 
which  will  be  equal  per  unit  of  length.  This  drop  can 
be  shown  by  connecting  the  terminals  of  a  delicate 
galvanometer,  generally  of  high  resistance,  to  the 
different  parts  of  the  wire.  The  deflection  of  the 
needle  will,  of  course,  be  greater  the  greater  the 
length  of  wire  between  the  two  points  touched. 

If,  now,  the  terminals  of  a  standard  voltaic  cell, 
whose  difference  of  potential  is  known,  be  connected 
so  as  to  oppose  the  current  taken  from  the  potentiom- 
eter wire,  and  the  contacts  be  slid  along  the  po- 
tentiometer wire  until  no  deflection  of  the  needle  is 
observed,  the  drop  of  potential  between  these  points 
on  the  potentiometer  wire  will  be  equal  to  the  differ- 
ence of  potential  of  the  standard  cell.  In  this  way 
the  wire  is  calibrated. 

Suppose,  now,  it  is  desired  to  measure  the  dif- 
ference of  potential  between  two  points  a  and  b, 
on  the  wire  C,  through  which  a  current  is  passing 
in  the  direction  of  the  arrow.  Connect  the  points  b 


50  ELECTRICAL  MEASUREMENTS. 

and  d,  with  a  galvanometer  G,  of  high  resistance, 
and  the  points  a  and  r,with  a  conductor ;  now  slide 
c,  toward  or  from  d,  until  the  galvanometer  shows 
no  deflection.  The  difference  of  potential  between 
a  and  I  is  then  equal  to  that  between  c  and  d. 

The  form  of  electrometer  shown  in  Fig.  24  is 
called  a  capillary  electrometer.  In  this  instrument 
a  horizontal  glass  tube,  which  is  filled  with  mercury 
and  has  a  drop  of  dilute  sulphuric  acid  at  B,  has 
its  ends  connected  with  two  vessels  M  and  N,  also 


FIG.  24.— CAPILLARY  ELECTROMETER. 

filled  with  mercury.  If  the  points  whose  difference 
of  potential  are  to  be  determined  are  connected  by 
means  of  conductors  with  the  vessels  JSfand  N,  a 
current  passes  through  the  capillary  tube  and  a 
movement  of  the  drop  of  acid  takes  place  toward  the 
negative  pole.  Provided  the  difference  of  potential 
does  not  exceed  two  volts,  the  amount  of  this  move- 
ment is  directly  proportional  to  the  difference  of  po- 
tential. 

A  ready  means  for  obtaining  a  known  difference  of 


MEASUREMENT  OF  ELECTROMOTIVE  FORCE.     51 

potential,  either  for  measuring  an  unknown  differ- 
ence of  potential  by  opposing  such  known  difference 
of  potential,  or  for  the  purpose  of  calibrating  an  in- 
strument, and  thus  determining  the  value  of  its  de- 
flections, is  found  in  various  standard  voltaic  cells. 
Ordinarily  constructed  voltaic  cells  would  be  im- 
practicable for  such  purposes.  With  standard  cells, 
if  especial  care  is  taken  to  avoid  polarization,  and 


W 

FIG.  25.— CLARK'S  STANDARD  CELL. 

the  circuit  of  the  cell  is  closed  but  for  a  short  time, 
the  electromotive  force  it  produces  is  practically 
constant.  Standard  voltaic  cells  are  made  in  a  great 
variety  of  forms. 

In  Fig.  25  is  shown  a  well-known  form  of  stand- 
ard voltaic  cell  devised  by  Latimer  Clark,  known  as 
the  H-form  of  cell,  from  the  shape  of  a  vessel  C  C 


52  ELECTRICAL  MEASUREMENTS. 

connected  by  the  cross  tube.  The  left-hand  vessel 
contains  at  A,  an  amalgam  of  pure  zinc  ;  the  right- 
hand  vessel  contains  at  M,  a  mass  of  mercury 
covered  with  pure  mercurous  sulphate.  Both  vessels 
are  then  filled  up  to  the  level  of  the  cross  tube  with 
a  saturated  solution  of  zinc  sulphate  to  which  a  few 


FIG.  26.— RAYLEIGH'S  FORM  OF  CLARK'S  STANDARD  CELL. 

crystals  of  the  salt  are  generally  added.  Tightly 
fitting  corks  prevent  loss  by  evaporation.  Wires 
W,  W,  fused  into  the  bottom  of  the  vessels,  serve 
as  the  terminals  of  the  cell. 

In  Fig.  26  is  shown  Rayleigh's  form  of  Clark's  stand- 


MEASUREMENT  OF  ELECTROMOTIVE  FORCE.     53 

ard  cell.  The  electrodes  pass  respectively  through  the 
bottom  and  top  of  a  glass  test-tube.  On  the  bottom  of 
the  cell  is  placed  a  layer  of  mercury  on  top  of  which 
is  placed  a  layer  of  mercurous  sulphate  paste  that  is 
rendered  sufficiently  semi-fluid  by  mixture  with  zinc 
sulphate  to  assume  an  approximately  level  surface. 


FIG.  27.— FLEMING'S  STANDARD  CELL. 

The  zinc  is  connected  to  the  upper  electrode  and  is 
immersed  in  this  semi-fluid  paste. 

Fig.  27  shows  Fleming's  standard  voltaic  cell. 
The  U-tube  is  connected  by  means  of  taps  with  two 
vessels  filled  respectively  with  chemically  pure  solu- 
tions of  copper  sulphate  of  the  specific  gravity  of  1.1 


54  ELECTRICAL  MEASUREMENTS. 

at  15°  C.,  and  zinc  sulphate  of  the  specific  gravity 
of  1.4  at  15°  C. 

To  use  the  cell,  the  zinc  rod  Zn,  connected  with 
the  wire  passing  through  the  rubber  stopper,  isplaced 
in  the  left-hand  branch.  The  tap  at  A,  is  opened  and 
the  entire  U-tube  is  filled  with  the  denser  zinc  sul- 
phate solution.  The  tap  at  6',  is  then  opened,  and  the 
liquid  in  the  right-hand  branch,  above  the  tap,  dis- 


FIG.  28.— LODGE'S  FORM  OF  DANIELL'S  CELL. 

charged  into  the  lower  vessel,  but  from  this  point 
only. 

The  tap  C,  is  then  closed  and  B,  is  opened,  and 
the  lighter  copper  sulphate  is  allowed  to  fill  the 
right-hand  branch  above  the  tap  C.  The  copper  rod 
Cu,  fitted  to  a  rubber  stopper  and  connected  to  a 
conducting  wire,  is  then  place'd  in  the  copper  solu- 
tion. Tubes  at  L  and  J/,  are  provided  for  the  recep- 


MEASUREMENT  OF  ELECTROMOTIVE  FORCE.      55 

tion  of  the  zinc  and  copper  rods  when  not  in  use. 
The  copper  rod  is  prepared  for  use  by  freshly  electro- 
plating it  with  copper.  The  electromotive  force  of 
this  cell,  when  in  proper  action,  is  1.074  volts. 
If  the  line  of  demarkation  between  the  two  liquids 
is  not  sharp,  the  arms  of  the  vessel  are  emptied,  and 
fresh  liquid  is  run  in. 

Lodge's  form  of  DaiiielFs  standard  cell  is  shown 
in  Fig.  28. 

Through  a  tube  T,  in  a  wide-mouthed  bottle,  is 
passed  the  glass  tube,  in  the  mouth  of  which  is 
placed  a  zinc  rod.  A  small  test  tube  t,  containing 
crystals  of  copper  sulphate,  is  fastened  to  the  bottom 
of  the  tube  T,  by  means  of  a  string  or  rubber  band. 
The  uncovered  end  of  a  gutta-percha  insulated  wire 
projects  at  the  bottom  t,  through  a  tube  in  a  tightly 
fitting  cork,  and  forms  the  copper  electrode.  The 
bottle  is  filled  with  a  solution  of  zinc  sulphate. 

The  internal  resistance  of  this  form  of  standard 
cell  is  so  high  that  it  is  only  employed  for  use  with 
measurements  employing  zero  methods  or  with  a 
condenser. 

In  any  form  of  standard  voltaic  cell  great  care 
must  be  taken  or  otherwise  appreciable  differences  in 
the  electromotive  force  will  result.  When  sulphate 
of  copper  is  used  care  must  be  taken  to  prevent  the 


56  ELECTRICAL  MEASUREMENTS. 

copper  from  being  deposited  on  the  zinc.  The  tem- 
perature should  be  kept  as  nearly  constant  as  pos- 
sible. The  electrolytes  should  be  pure  and  kept  at 
the  same  density.  Where  saturated  solutions  are 
employed,  as  in  the  case  of  the  zinc  sulphate  em- 
ployed in  Clarke's  H-form  of  cell,  care  should  be 
taken  to  prevent  the  solution  from  becoming  super- 
saturated. 

A  solution  is  saturated  with  a  soluble  salt  when  it 
contains  as  much  as  it  can  hold.  When  a  saturated 
solution  is  cooled  it  deposits  some  of  the  salt,  the 
liquid  still  remaining  saturated.  If,  however,  the 
liquid  be  closed  from  the  air  and  cooled  without 
shaking,  the  deposit  of  crystals  may  not  occur,  and 
the  solution  then  becomes  super-saturated. 


MEASUREMENT  OF  ELECTROMOTIVE  FORCE.     57 


EXTRACTS  FROM  STANDARD   WORKS. 

Gray,  in  his  work  entitled  "The  Theory  and 
Practice  of  Absolute  Measurements  in  Electricity 
and  Magnetism,"*  in  describing  electrometers,  says 
on  page  252  : 

An  electrometer  has  been  defined  as  an  instrument  for 
measuring  differences  of  electric  potential  by  means  of  the 
effects  of  electrostatic  force.  It  consists  essentially  of  two 
conductors,  between  which  is  established  the  difference  of 
potential  which  it  is  desired  to  measure.  The  electrostatic 
force  set  up  produces  motion  of  the  parts  of  one  of  these 
conductors  relatively  to  one  another,  or  motion  of  the  con- 
ductor as  a  whole  relatively  to  the  other  conductor  ;  and 
from  this  motion,  or  from  the  mechanical  force  which 
must  be  applied  to  restore  and  maintain  equilibrium  in  the 
configuration  of  zero  electrification,  the  difference  of  po- 
tentials is  inferred.  We  shall  call  this  conductor  the  Indi- 
cator of  the  instrument. 

When  the  instrument  contains  within  itself  a  means  of 
comparing  the  electric  force  called  into  play  with  other 
forces  known  in  amount,  as,  for  example,  the  force  of 
gravity  on  a  given  mass,  or  the  elastic  reaction  of  a 
stretched  spring,  it  gives  directly  by  its  indications  the 

*  "  The  Theory  and  Practice  of  Absolute  Measurements  in  Elec- 
tricity and  Magnetism,"  by  Andrew  Gray,  M.  A.,  F.R.S.  London: 
Macmillan  &  Co.  1888.  518  pages,  105  illustrations.  Price  $3.75. 


58  ELECTRICAL  MEASUREMENTS. 

value  in  absolute  electrostatic  units  of  the  difference  of 
potential  measured,  and  is  called  an  absolute  electrometer. 

When  the  instrument  gives  only  comparisons  of  the  elec- 
trostatic forces  with  other  forces,  the  amount  of  which  it 
does  not  itself  contain  any  means  of  determining,  its  indi- 
cations can  only  be  obtained  in  absolute  units  by  a  compar- 
ison with  those  of  an  absolute  electrometer. 

When  the  mode  of  variation  of  these  undetermined 
forces  is  known  and  remains  constant,  one  such  accurate 
comparison  is  sufficient  to  allow  a  coefficient  to  be  deter- 
mined by  which  results  proportional  to  measured  differ- 
ence of  potential  must  be  multiplied  fcr  reduction  to  ab- 
solute measure.  The  coefficient  is  called  the  constant  of 
the  instrument. 

In  a  volume  on  "  Primary  Batteries/'  *  on  page 
90,  Carhart  thus  refers  to  the  objections  that  exist  as 
regards  Lord  Rayleigh's  form  of  Clark's  Standard 
cell: 

The  objections  to  Lord  Rayleigh's  form  of  the  Clark  nor- 
mal element  are:  (1)  the  temperature  coefficient  is  high  and 
apparently  variable  :  (2)  it  is  not  constructed  in  such  manner 
as  to  keep  the  zinc  and  metallic  mercury  out  of  contact ; 
(3)  the  contact  of  the  zinc  and  mercurial  salt  permits  of 
local  action  whereby  zinc  replaces  mercury. 

Respecting  the  first  objection,  the  method  to  be  pursued 
in  reducing  the  temperature  coefficient  is  suggested  by  the 

*" Primary  Batteries,"  by  Henry  S.  Carhart,  A.M.  Boston: 
Allyn&  Bacon.  1891.  193  pages,  67  illustrations.  Price  $1.50. 


MEASUREMENT  OF  ELECTROMOTIVE  FORCE.     59 

fact,  now  well  known,  that  the  E.  M.  F.  decreases  with  an 
increase  in  the  density  of  the  zinc  sulphate  solution. 
Hence,  if  the  solution  is  saturated  at  30'  or  40%  upon  a 
lowering  of  temperature  the  excess  crystallizes  out  wi*h  a 
decrease  of  density.  The  reverse  process  takes  place  with 
rise  of  temperature,  with  the  additional  disadvantage  that 
time  is  required  for  the  diffusion  of  the  redissolved  salt. 
The  temperature  coefficient  in  such  a  cell  is  therefore  made 
up  of  two  parts  ;  one  a  real  temperature  effect,  the  other 
a  secondary  change  resulting  from  a  variability  in  the 
density  of  the  zinc  sulphate  solution.  A  rise  of  tempera- 
ture lowers  the  E.  M.  F.  by  increasing  the  density  of  the 
solution  in  addition  to  the  direct  primary  effect  of  the  tem- 
perature change. 

The  slowness  of  diffusion  when  the  temperature  rises 
makes  the  coefficient  for  a  rapid  change  of  temperature 
smaller  than  for  a  slow  one.  Thus  Prof.  Threlfall,  in- 
vestigating Clark  cells  made  in  accordance  with  Lord 
Rayleigh's  directions,  found  the  coefficient  to  be  .000402 
for  a  rapid  rise  of  temperature  from  21°  to  34°  C.  This  is 
less  t'  an  half  the  value  found  by  Lord  Rayleigh  between 
the  same  temperatures. 

The  magnitude  of  the  temperature  coefficient  depends, 
then,  upon  the  temperature  at  which  the  zinc  salt  is  satu- 
rated, and,  because  of  diffusion,  upon  the  rapidity  of  the 
temperature  change.  To  obviate  these  difficulties  the  zinc 
sulphate  should  be  saturated  at  some  definite  temperature 
lower  than  any  at  which  the  cell  is  to  be  used.  The  tem- 
perature selected  by  the  writer  is  that  of  melting  ice.  *  *  * 

The  other  two  objections  urged  against  the  usual  form  of 


60  ELECTRICAL  MEASUREMENTS. 

Clark  cell  are  founded  chiefly  on  the  local  action  taking 
place  when  the  zinc  and  mercurial  salt  are  in  contact. 
Zinc  replaces  mercury  to  some  extent  when  in  contact  with 
a  salt  of  mercury.  With  the  oxide  of  mercury  this  action 
is  very  marked,  resulting  in  the  reduction  of  the  mercury 
and  oxidation  of  zinc.  The  same  replacement  process  goes 
on  with  mercurous  sulphate,  zinc  sulphate  being  formed  at 
the  expense  of  zinc  and  mercury  sulphate,  while  the  zinc 
is  amalgamated  with  the  reduced  mercury.  A  progressive 
change  in  the  density  of  the  solution  ensues,  entailing 
perhaps  a  rise  in  the  value  of  the  temperature  coefficient. 


III.— THE  MEASUREMENT  OF  ELECTRIC 
RESISTANCES. 


In  accordance  with  Ohm's  law,  when  the  differ- 
ence of  potential  and  the  resistance  of  any  circuit 
are  known,  the  current  strength  in  such  circuit,  or 
th|  number  of  coulombs  per  second,  can  be  readily 
calculated.  It  is  for  this,  and  for  many  other  rea- 
sons, a  matter  of  great  importance  to  be  able  to 
readily  determine  the  resistance  of  any  circuit,  or 
part  of  a  circuit. 

Various  methods  can  be  employed  for  measuring 
electric  resistances  ;  among  the  most  important  of 
these  are  the  following: 

(1.)  By  the  method  of  substitution. 

(2.)  By  a  comparison  of  the  deflections  of  a  gal- 
vanometer. 

(3.)  By  means  of  differential  galvanometers. 

(4.)  By  means  of  a  resistance  bridge  in  connec- 
tion with  a  box  of  resistance  coils. 

(5.)  By  the  indirect  method  of  measuring  the  cur- 
rent and  the  electromotive  force,  and  then  calcu- 

TTT 

lating  the  resistance  from  the  formula  R  =  -^-' 
(61) 


62  ELECTRICAL  MEASUREMENTS, 

In  the  method  of  substitution,  the  resistance  to 
be  measured  x,  Fig.  29,  a  box  of  resistance  coils  K 
and  a  galvanometer  G,  are  placed  in  series  in  a  cir- 
cuit with  a  voltaic  battery  B,  by  means  of  con- 
ductors that  are  sufficiently  thick  to  permit  their 
resistance  to  be  neglected.  The  deflection  of  the 
galvanometer  is  first  obtained  with  the  known  re- 
sistance x,  only  in  the  circuit  ;  that  is,  with  no  re- 
sistance in  the  box  B,  which  is  effected  by  inserting 
all  its  plug  keys. 

R 


FIG.  29.— SUBSTITUTION  METHOD. 

The  resistances,  is  then  cut  out  of  the  circuit  by 
placing  a  thick  copper  wire  m  m',  across  x,  so  as  to 
short-circuit  it.  Eesistances  are  then  unplugged  in 
the  box  R,  until  the  same  deflection  is  obtained  in 
the  galvanometer,  when,  provided  the  difference  of 
potential  of  the  battery  has  remained  constant,  the 
resistance  unplugged  will  equal  the  unknown  resist- 
ance. 

The  resistance  box  employed  in  the  above  method 


MEASUREMENT  OF  ELECTRIC  RESISTANCES.       63 

consists  generally  of  a  number  of  coils  of  wire,  the 
electric  resistance  of  which  is  accurately  known.  In 
order  to  prevent  the  magnetic  field  produced  by  such 
coils,  when  traversed  by  an  electric  current,  from 
affecting  the  needle  of  a  galvanometer  placed  near 
them,  they  are  wound  after  the  wire  which  forms 
them  has  been  doubled  or  bent  on  itself  in  two  equal 
lengths  so  that  the  two  halves  extend  parallel  with 
each  other. 


FIG.  30.— RESISTANCE  COILS. 

The  arrangement  of  the  coils  which  form  the  re- 
sistance, as  well  as  this  method  of  winding,  are 
shown  in  Fig.  30.  The  coils  C  C',  are  connected  to 
each  other  by  means  of  thick  pieces  of  brass  E,  E,  E, 
to  which  their  ends  are  soldered.  When  the  plug 
keys  are  placed  in  the  holes  or  spaces  left  at  S,  S,  be- 
tween the  contiguous  brass  pieces  E,  E,  E,  the  coils 
are  cut  out  of  the  circuit  by  short-circuiting.  When, 


64  ELECTRICAL  MEASUREMENTS. 

however,  the  keys  are  removed,  the  coils  are  placed 
in  the  circuit  by  what  is  technically  called  unplug- 
ging- 

In  order  to  avoid  changes  in  the  electric  resistance 
of  the  coils,  on  changes  in  temperature,  the  coils  are 
generally  made  of  German  silver  wire,  or,  preferably, 
of  platinoid,  or  an  alloy  of  platinum  and  silver,  the 
resistance  of  which  is  not  sensibly  affected  like  that 
of  most  other  metals  by  changes  in  temperature. 
Even,  however,  in  such  cases  it  is  necessary  not  to 
permit  the  current  to  flow  through  the  coils  for 
longer  than  a  few  minutes  at  a  time. 

The  method  of  determining  the  value  of  a  resist- 
ance by  a  comparison  of  the  deflection  of  galva- 
nometers is  based  on  the  fact  that  such  deflections 
are  proportional  to  the  current  passing,  and,  if  the 
electromotive  force  of  the  electric  source  employed 
is  constant,  that  the  current  which  passes  will  de- 
crease as  the  resistance  increases. 

The  method  of  determining  resistances  by  the  use 
of  differential  galvanometers  is  based  on  balancing 
the  deflection  of  the  needle  of  a  galvanometer  placed 
in  the  circuit  of  one  of  the  coils  in  which  the  un- 
known resistance  is  placed,  by  the  opposing  magnetic 
effects  of  the  other  coil  in  which  a  known  resistance 
is  placed. 


MEASUREMENT  OF  ELECTRIC  RESISTANCES.      65 

By  far  the  preferable  method  of  determining  re- 
sistances is  by  means  of  a  device  invented  by  Wheat- 
stone,  called  the  electric  bridge;  or,  as  it  was  orig- 
inally called,  the  electric  balance,  because  in  it  a 
known  resistance  is  balanced  against  an  unknown 
resistance. 

The  operation  of  the  electric  bridge  or  balance  is 
based  on  the  fact  that  no  current  will  flow  through  a 
conductor  the  terminals  of  which  are  connected  to 
points  that  are  of  the  same  difference  of  potential. 


Ztt    G 

FIG.  31.— ELECTRIC  BRIDGE. 

A  simple  form  of  electric  bridge  is  shown  in  Fig. 
31.  A,  B,  C,D,  are  electric  resistances  called  the 
arms  of  the  bridge.  Any  of  these  resistances  can  be 
determined,  provided  the  absolute  value  of  one  and 
the  relative  values  of  the  other  two  are  known. 

These  resistances  are  arranged  in  the  manner 
shown,  and  the  terminals  Zn  and  C,  of  the  voltaic 
battery,  are  connected  to  the  points  Q  and  P.  The 


66  ELECTRICAL  MEASUREMENTS. 

current  of  the  battery  then  flows  through  the  cir- 
cuit, branches  at  P,  part  flowing  through  the 
arms  D  and  C,  and  the  remainder  flowing  through 
the  arms  B  and  A.  These  tv/o  branches  unite  at  Q, 
and  return  to  the  battery.  A  sensitive  galvanome- 
ter, G,  is  placed  in  the  circuit  connecting  the  points 
Jfand  N.  The  name  bridge  was  derived  from  the 
fact  that  this  wire  or  conductor  bridges  or  connects 
the  points  M  and  N. 

When  a  current  passes  through  any  conductor,  a 
drop  or  fall  of  potential  occurs  that  is  proportional 
to  the  resistance.  When,  therefore,  the  current  from 
the  voltaic  battery,  Zn  C,  branches  at  P,  and  passes 
through  the  resistances  D  and  C,  and  B  and  A,  a 
drop  or  fall  of  potential  occurs  along  the  paths  D 
and  C,  and  B  and  A,  proportional  to  their  resistances. 
If  the  points  M  and  N,  that  are  connected  by  the 
bridge  wire,  are  at  the  same  difference  of  potential,  no 
current  will  pass  ;  if,  however,  they  are  at  a  differ- 
ent potential,  a  current  will  pass  from  H,  to  N,  if  J/", 
is  at  the  higher  potential,  and  in  the  opposite  direc- 
tion, from  N,  to  M,  if  M,  is  at  the  lower  potential, 
and  the  needle  of  the  galvanometer  will  be  deflected 
according  to  the  direction  in  which  the  current  flows. 
If,  therefore,  the  known  resistances.  A,  C  and  B,  are 
so  proportioned  to  the  value  of  the  unknown  resist- 


MEASUREMENT  OF  ELECTRIC  RESISTANCES.     67 

ance  D,  that  no  current  passes  through  the  galvan- 
ometer G,  between  the  points  M  and  N,  then  such 
points  are  at  the  same  difference  of  potential,  and 
since  the  fall  of  potential  is  proportional  to  the  re- 
sistance, it. follows  that 

AiBr.CiD, 
A  x  D  =  B  x  C, 

or  D 

If,  then,  the  values  A,  B  and  C,  are  known,  the 
value  of  D,  can  be  readily  calculated. 

By  making  the  value  — r    some  simple   ratio,  the 

value  of  D,  is  easily  obtained  in  terms  of  C. 

The  resistances  A,  B  and  C,  may  consist  of  coils 
of  wire  whose  resistances  are  unknown. 

There  are  two  forms  of  Wheatstone  bridge  ;  name- 
ly, the  box  form  and  the  sliding  form.  In  the  box 
form,  the  arms  of  known  resistance  of  the  bridge 
consist  of  resistance  coils.  In  the  sliding  form  one 
of  the  known  resistances  consists  of  a  resistance 
coil,  and  the  other  two  of  a  uniform  wire  or  con- 
ductor over  which  a  sliding  contact  moves  so  as  to 
place  different  lengths  of  the  conductor  on  different 
sides  o£  the  slide,  and,  therefore,  in  different  arms 
of  the  bridge. 


ELECTRICAL  MEASUREMENTS. 


The  box  form  of  bridge  is  shown  in  perspective  in 
Fig.  32  and  in  plan  in  Fig.  33. 

It  will  be  noticed  that  the  bridge  arm  correspond- 
ing to  the  resistances  A  and  B,  of  Fig.  31,  consists 


FIG.  32.— Box  BRIDGE. 

of  resistance  coils  of  10, 100,  1,000  ohms  each,  called 
the  proportionate  coils.  The  arm  corresponding  to 
the  resistance  C,  of  the  same  figure,  is  composed  of 
separate  resistances  1,  2,  2,  5,  10,  10,  20,  50,  100, 
100,  200,  500,  1,000,  1,000,  2,000,  and  5,000  ohms 


FIG.  33.— Box  BRIDGE. 


The  following  are  the  connections:  the  galvanom- 
eter is  inserted  between  q  and  r,  Fig.  34;  the  un- 
known resistance  between  the  z  and  r,  and  the  battery 


MEASUREMENT  OF  ELECTRIC  RESISTANCES.      69 

is  connected  to  x  and  z.  A  definite  proportion  being 
taken  for  the  value  of  the  proportionate  coils,  such, 
for  example,  as  10  on  one  side,  and  100  on  the  other, 
resistances  are  inserted  in  the  arm  D,  until  the  gal- 
vanometer 6r,  shows  no  deflection. 

The  similarity  between  these  connections  and 
those  shown  in  Fig.  31,  will  be  recognized  from  an 
inspection  of  Fig.  34.  The  arms  A  and  B,  of  Fig. 
31,  correspond  to  the  arms  q  x  and  qz,  of  Fig.  34  ; 
the  arm  G,  of  Fig.  31,  corresponds  to  the  arm  x  r, 


FIG.  34.  -ELECTRIC  BALANCE. 

of    Fig.    34,   and  D,  to   the   unknown    resistance. 
Then  as  before  we  have  : 


A  :  B  ::  C:  D,  or  A  x  D  =  B  x  C.  .'.  D  = 

The  advantage  of  the  simplicity  of  the  ratios  A 
and  B,  or  10,  100  and  1,000  of  the  bridge  box,  will 
therefore  be  manifest.  The  battery  terminals  may 


70  ELECTRICAL  MEASUREMENTS. 

be  connected  to  q  and  r,  and  the  galvanometer  ter- 
minals to  x  and  z,  without  disturbing  the  above  pro- 
portions. 

In  the  slide  form  of  bridge,  as  shown  in  Fig.  30, 
the  proportionate  arms  are  formed  of  a  single  thin 
wire  of  high  resistance  and  uniform  diameter, 
formed  of  German  silver  or  platinoid. 

The  slide  contact  key  moves  over  the  wire,  one 
terminal  of  the  key  being  connected  with  the  gal- 
vanometer and  the  other,  when  the  key  is  depressed, 
with  the  wire.  From  the  uniform  diameter  of  the 


r~&  &  

f                0t 

FIG.  35.— SLIDIC  BRIDGE. 

wire,  the  resistance  on  either  side  of  the  key  will  be 
directly  proportional  to  the  length  of  the  wire.  A 
graduated  scale  placed  under  the  wire  serves  to 
measure  its  length.  A  thick  metal  strip  connected 
with  the  slide  wire  has  four  gaps  at  P,  Q,  R  and  8. 
When  in  ordinary  use  the  gaps  at  P  and  8,  are 
either  connected  by  stout  strips  of  conducting  mate- 
rial, or  by  known  resistances  ;  in  the  latter  case  they 
act  as  ungraduated  extensions  of  the  slide  wire,  and, 


MEASUREMENT  OF  ELECTRIC  RESIST  A  NCES.      7  J 


Fio.  36.— SLIDE  BRIDGE* 


like  lengthening  the  slide  vire,  increase  the  sensibil- 
ity of  the  bridge. 


72  ELECTRICAL  MEASUREMENTS. 

The  unknown  resistance  is  then  inserted  in  the 
gap  at  Q,  and  the  known  resistance,  generally  a 
resistance  box,  in  the  gap  at  R.  The  galvanometer 
has  one  of  its  terminals  connected  to  the  metal  strips 
between  Q  and  R,  and  its  other  terminal  to  the 
sliding  key.  The  battery  terminals  are  connected 
to  the  metal  strips  between  P  and  Q,  and  R  and  S, 
respectively. 

These  connections  are  more  clearly  seen  in  the 
form  of  bridge  shown  in  Fig.  36.  The  slide  wire 
iv  w,  consists  of  three  separate  wires  each  a  meter  in 
length,  and  so  arranged  that  either  only  one  wire 
or  two  in  series  can  be  used.  The  circuit  being  now 
arranged  as  shown,  the  sliding  key  is  moved  until 
no  current  flows  through  the  galvanometer  when  the 
key  is  depressed. 

The  slide  form  of  bridge  is  not  entirely  satisfac- 
tory, since  the  uncertainty  of  the  spring  contact 
causes  a  lack  of  correspondence  between  the  con- 
tact and  the  points  of  the  scale  on  which  the  index 
rests. 

The  loss  of  uniformity  in  the  diameter  of  the 
wire,  due  to  constant  use,  also  causes  a  lack  of  cor- 
respondence between  the  resistance  of  the  wire  and 
its  length.  With  care,  however,  very  accurate  re- 
sults can  be  obtained  by  the  slide  bridge. 


MEASUREMENT  OF  ELECTRIC  RESISTANCES.       73 

For  the  purpose  of  standardizing  resistance  coils, 
a  standard  coil  of  some  known  value  is  necessary.  A 
form  of  standard  ohrn,  issued  by  the  Committee  of 
Electrical  Standards  in  England,  is  shown  in  Fig. 
37.  A  coil  of  platinum  silver  wire,  insulated  by 
silk  covering  and  melted  paraffine,  is  placed  at  B, 
and  the  space  above  it  at  A,  is  filled  with  paraffine 
except  at  t,  where  an  opening  is  left  for  the  insertion 


FIG.  37.— STANDARD  OHM. 

of  a  thermometer.     The  ends  of  the  resistance  coil 
are  soldered  to  thick  copper  rods  r  rf,  as  shown. 

In  the  resistance  box  already  described  the  ad- 
justability of  the  resistance  is  obtained  by  means  of 
unplugging  coils  of  various  values  until  the  required 
resistance  is  introduced  into  the  circuit.  Wheatstone 
devised  an  apparatus  in  the  shape  of  an  adjustable 
resistance  or  rheostat  of  the  form  shown  in  Fig.  38. 


74  ELECTRICAL  MEASUREMENTS. 

This  instrument  is  suitable  for  rough  work,  but  is 
not  very  satisfactory  where  great  accuracy  is  re- 
quired. It  consists,  as  shown,  of  two  parallel  cylin- 
ders A  and  B,  formed  respectively  of  conducting  and 
non-conducting  material.  The  resistance  wire, 
which  is  a  bare  wire,  can  be  wound  from  the  surface 
of  the  non-conducting  cylinder  B,  to  the  conducting 
cylinder  A.  Only  the  wire  on  B,  is  introduced  into 


FIG.  33.— WHEATS-TONE'S  RHEOSTAT. 


the  circuit,  since  the  bare  wire  on  the  conducting 
cylinder  A,  is  short  circuited  by  the  metallic  cylinder. 

As  a  ready  means  for  measuring  the  resistance  of 
a  circuit  through  which  a  current  of  electricity  is 
passing  Ayrton  has  devised  an  instrument  called  an 
ohmmeter,  which  shows  directly,  by  the  deflection  of 
a  magnetic  needle,  the  resistance  in  any  part  of  a 
circuit. 

The  construction  of  the  apparatus  will  be  under- 


MEASUREMENT  OF  ELECTRIC  RESISTANCES.      75 

stood  from  an  inspection  of  Fig.  39.  Two  coils  of 
wire,  C  C  and  c  c,  formed  respectively  of  short  thick 
wire  and  long  thin  wire,  are  placed  at  right  angles 
to  each  other,  and  act  on  a  soft  iron  needle  placed 
as  shown.  The  short  thick  coil  CC,  is  connected  in 
series  to  the  conductor  0,  whose  resistance  is  to  be 
measured.  The  long  thin  coil  c  c,  of  known  high 
resistance,  is  placed  as  a  shunt  to  0. 

Under  these  circumstances  the  action  of  the  needle 
is  due  to  the  ratio  of  the  difference  of  potential  at 


FIG.  39.— AYKTON'S  OHMMEIER. 

the  terminals  of  the  unknown  resistance  and  the 
current  strength  in  the  thick  coil,  or  R  C  =  E,  as 
may  be  deduced  from  Ohm's  law. 

The  coils  are  so  proportioned  that  when  the  current 
flows  through  the  short  thick  wire  it  moves  the 
needle  to  the  zero  of  the  scale,  while  the  long  thin 
wire  produces  a  deflection  directly  proportional  to 
the  resistance. 

When  a  conductor  is  in  the  form  of  a  bare  wire  of 
uniform  area  of  cross  section,  the  resistance  of  a 


76  ELECTRICAL  MEASUREMENTS. 

given  length  of  such  conductor  can  be  readily  cal- 
culated from  a  knowledge  of  its  diameter,  provided, 
of  course,  the  specific  resistance  of  such  material — 
that  is,  the  resistance  of  a  unit  length  of  unit  area 
of  cross  section — is  known. 

In  order  to  readily  determine  the  diameter  of  a 
wire  so  as  to  calculate  the  resistance  of  a  given 
length  of  such  wire,  various  instruments  called  wire 
gauges  are  employed. 


Fia.  40.— ROUND  WIRE  GAUGE. 

Fig.  40  shows  a  form  of  wire  gauge  called  the 
round-wire  gauge.  In  this  gauge  notches  of  varying 
width,  cut  in  the  edges  of  a  circular  plate  of  steel, 
serve  to  approximately  measure  the  diameter  of  a 


MEASUREMENT  OF  ELECTRIC  RESISTANCES.       77 

wire  which  is  passed  lengthwise  through  the  open- 
ings. Numbers  indicating  the  different  sizes  of  the 
wire  are  affixed  to  each  of  the  openings. 

Another  form  of  wire  gauge,  called  the  microm- 
eter wire  gauge,  is  shown  in  Fig.  41. 

The  wire  to  be  measured  is  placed  between  the 
fixed  support  B,  and  the  movable  end  C,  of  a  long 
screw,  which  accurately  fits  a  threaded  tube  a.  A 
thimble  D,  provided  with  a  milled  head,  fits  Over  the 


FIG.  41.— VKRNIER  WIRE  GAUGE. 

screw  C,  and  is  attached  to  the  upper  part.  The 
lower  circumference  of  D  is  divided  into  a  scale  of 
20  equal  parts.  The  tube  a  is  marked  in  divisions 
equal  to  the  pitch  of  the  screw.  Every  fifth  of  these 
divisions  is  marked  as  a  larger  division. 

The  principle  of  operation  of  the  gauge  is  as  fol- 
lows: Suppose  the  screw  has  50  threads  to  the  inch ; 
the  pitch  of  the  screw,  or  the  distance  between  two 


78  ELECTRICAL  MEASUREMENTS. 

contiguous  threads,  is,  therefore,  -fa,  or  -°2>  of  an 
inch. 

One  complete  turn  of  the  screw  will,  therefore, 
advance  the  sleeve  D,  over  the  scale  a,  the  .02  of  an 
inch.  If  the  screw  is  only  moved  through  one  of 
the  twenty  parts  marked  on  the  end  of  the  thimble 
or  sleeve  parts,  or  the  -fa  of  a  complete  turn,  the  end 
C,  advances  toward  B,  the  ^0-  of  -fa,  that  is,  -j-gVu-j 
or  .001,  inch. 

Suppose,  now,  a  wire  is  placed  between  B  and  C, 
and  the  screw  advanced  until  the  wire  fairly  fills  the 
space  between  B  and  C,  and  the  reading  shows  two 
of  the  larger  divisions  of  the  scale  a,  three  of  the 
smaller  ones  and  three  on  the  end  of  the  sleeve  D, 
then 

Two  large  divisions  of  scale  a  =  .2  inch. 

Three  smaller  divisions  of  scale  a  =  .06  inch. 

Three  divisions  on  circular  scale  on  D  =  .003 
inch. 

Diameter  of  wire  =  .263  inch. 

In  the  self  registering  wire  gauge  the  apparatus  is 
arranged  to  give  directly  without  calculation  the 
exact  diameter  of  the  wire  to  be  measured. 

A  form  of  self-registering  wire  gauge  is  shown  in 
Fig.  42.  The  wire  or  plate  is  inserted  in  the  gap 
between  a  fixed  and  a  movable  plate.  The  diam- 


MEASUREMENTS  OF  ELECTRIC  RESISTANCES      79 

eters  of  the  wire  or  plate  that  is  being  measured  are 
then  read  off,  shown  on  one  side  of  the  gauge,  and 
the  gauge  numbers  on  the  other. 

In  making  calculations  of  the  resistance  of  any 
circuit  it  must  carefully  be  remembered  that  changes 
in  temperature  produce  changes  in  resistance.  The 
character  of  such  changes  can  be  summarized  as  fol- 
lows: 


FIG.  42.— WIRE  AND  PLATE  GAUGE. 
(1.)  The  electric  resistance  of  metallic  conductors 
increases  as  the  temperature  rises.  The  resistance 
of  the  carbon  conductor  of  an  incandescent  lamp 
decreases  as  the  temperature  rises.  Its  resistance 
decreases  when  it  is  raised  to  incandescence,  such 
decrease  amounting  to  about  three-eighths  of  its  re- 
sistance when  cold. 


SO 


ELECTRICAL  MEASUREMENTS. 


Roughly  speaking  the  increase  in  temperature  is 
proportional  to  the  temperature.  In  reality,  the  re- 
sistance of  all  metals  except  mercury  increases  more 
rapidly  than  the  temperature. 


CHEMICALLY  PURE  SUBSTANCES  ARRANGED  IN  ORDER  OF  INCREAS- 
ING RESISTANCE  FOR  THE  SAME  LENGTH  AND  SECTIONAL  AREA. 
LEGAL  MICROHM. 


NAME  OF  METAL. 

Resistance  in 
microhms  at 
0°  C. 

Relative 
resist- 
ances. 

Cubic 
centi- 
metre. 

1.504 
1.598 
1.634 
1.834 
2.058 
2.094 
2.912 
5.626 
9.057 
9.716 

10.87 
12.47 
13.21 
19.63 
20.93 

24.39 
35.50 
94.32 
131.2 

Cubic 
inch. 

Silver,  annealed  

0.5921 
0.6292 
0.6433 
0.6433 
0.8102 
0.8247 
1.147 
2.215 
3.565 
3.825 

4.281 
4.907 
5.202 
7.728 
8.240 

9.603 
13.98 
37.15 
51.65 

!063 
.086 
.086 
.369 
.393 
.935 
3.741 
6.022 
6.460 

7.228 
8.285 
8.784 
13.05 
13.92 

16.21 
2H.RO 
62.73 

87.23 

Copper,  hard  drawn  

Gold,  hard  drawn  

Platinum  ,  annealed  

Gold  -silver  alloy  (3  oz.  gold,  1  oz.  silver) 
hard  or  annealed  

Lead,  pressed  

Platinum-silver  alloy  (1  oz.  platinum,  2  oz. 
silver),  hard  or  annealed  

Bismuth,  pressed  

Dr.  Matthiessen's  investigations  as  to  the  resist- 
ance of  various  metals,  and  the  effects  of  temper- 
ature thereon,  give  the  resistance  of  certain  cubic 


MEASUREMENTS  OF  ELECTRIC  RESISTANCES.       81 

volumes,  such  for  example,  as  one  cubic  centi- 
metre, or  one  cubic  inch  of  such  metals  in  michrorns. 

In  the  preceding  table,  from  Ayrton,  calculated 
from  Matthiessen's  results,  the  resistance  in  microhms 
is  given  of  a  cubic  centimetre  and  a  cubic  inch  re- 
spectively of  the  various  metals.  In  the  last  column 
will  be  found  the  relative  resistance  as  compared 
with  silver.  The  resistance  is  measured  laterally 
across  the  cube  from  one  face  to  the  opposite  face. 

(2.)  The  resistance  of  an  electrolyte  decreases  as 
temperature  increases. 

(3.)  The  resistance  of  di-electrics  or  non-conduct- 
ors decreases  as  the  temperature  increases. 

The  resistance  of  a  wire  is  directly  proportional 
to  its  length,  inversely  proportional  to  the  area  of 
cross-section,  or  to  the  square  of  the  diameter,  and 
depends  on  the  material  of  which  the  wire  is  formed  ; 
calling  R,  the  resistance,  I,  the  length,  d,  the  diame- 
ter, and  K,  a  const  ant,  varying  with  the  material, 
2 


then  R  =  (Ji)  K. 


In  the  following  table  from  Jenkin  the  conduct- 
ing power,  or  resistance  in  ohms,  is  given  fora  num- 
ber of  metals.  The  conducting  power  is  compared 
with  that  of  pure  hard  drawn  silver  wire,  which  is 
taken  as  one  hundred. 


ELECTRICAL  MEASUREMENTS. 


TABLE    OF    CONDUCTING    POWERS    AND    RESISTANCES    IN  OHMS. 


0 

2  ° 

0  M 

c  d 

®  ' 

„.   • 

*H 

§§ 

ft 

£  a 

Ps 

|||o 

09 

*1 

"il 

^     K> 

rt°3 

|Sg| 

NAMES  OF  METALS 

ad 

i! 

||| 

"o 

<u  Ms: 

aj_o»Ij 

Us 

|g| 

p» 

"tfi^-*3 

g>gg 

5C-o 

!<2& 

'is§ 

111 

£ 

K 

tf 

OH 

•< 

Silver,  annealed. 

0.2214 

0.1544 

9.936 

o.'1937 

0  377 

Silver,  hard  drawn 

100.00 

0.2421 

0.1689 

0.02103 

Copper,   annealed.. 
Copper,  hard  drawn 

'"99:55 

0.2064 
0.210! 

0.144 
0.1469 

g^'lS 
9.94 

0.02057 
0.02104 

0.388 

Gold,  annealed  

0.5849 

0.408 

12.52 

0.0265 

"6:355' 

Gold,  hard  drawn  .  . 

'"77:96 

0.295 

0  415 

12.74 

0.02697 

Aluminium  ,     an- 

nealed          

0.06822 

0.05759 

17.72 

0.03751 

Zinc,  pressed  

29.02 

0.571 

0.3983 

32,22 

0.07244 

*  '6'sfi.V 

Platinum,  annealed 

3.526 

2.464 

55.09 

0.1166 

Iron,  annealed  

16  81 

1.2425 

0.7522 

59.40 

0.1251 

Nickel,    annealed  .  . 
Tin,  pressed  

13:11 

12.36 

1.0785 
1.317 

0.8666 
0.9184 

75.78 
80.36 

0.1604 
0.1701 

'6:365' 

Lead,  pressed  

8.32 

3236 

2.257 

119.39 

0.2527 

0.387 

Antimony,  pressed. 
Bismuth,  pressed... 

4.62 
1.24 

3.321 
5.054 

2.3295 
3.5>5 

216.0 
798.0 

0.4571 
1  689 

0.389 
0.354 

Mercury,  liquid.... 

18.74 

13.071 

600.0 

1.27 

0.072 

Platinum  silver,  al- 

loy, hard    or  an- 

nealed   



4.243 

2.959 

143.35 

0.3140 

0.031 

German  silver,  hard 

or  annealed 

2.652 

1.85 

127.32 

0.2695 

0.044 

Gold,    silver  alloy, 

hard  or  annealed.. 



2.391 

1.668 

66.1 

0.1399 

0.065 

-Jenkin. 


MEASUREMENT  OF  ELECTRIC  RESISTANCES.      83 


EXTRACTS  FROM  STANDARD  WORKS. 

Kempe  in  his  "Handbook  of  Electrical  Testing,"* 
on  page  10,  speaks  of  resistance  coils  as  follows  : 

The  essential  points  of  a  good  set  of  resistance  coils  are, 
that  they  should  not  vary  their  resistance  appreciably 
through  change  of  temperature,  and  that  they  should  be 
accurately  adjusted  to  the  standard  units,  which  adjust- 
ment ought  to  be  such  that  not  only  should  each  individual 
coil  test  according  to  its  marked  value,  but  the  total  value 
of  all  the  coils  together  should  be  equal  to  the  numerical 
sum  of  their  marked  values.  It  will  be  frequently  found 
in  imperfectly  adjusted  coils  that  although  each  individual 
coil  may  test,  as  far  as  can  be  seen,  correctly,  yet  when 
tested  altogether  their  total  value  will  be  one  or  two  units 
more  or  less  than  the  sum  of  their  individual  values  ;  be- 
cause, although  an  error  of  a  fraction  of  a  unit  may  not  be 
perceptible  in  testing  each  coil  individually,  yet  the  ac- 
cumulated error  may  be  comparatively  large. 

The  wire  of  the  coils  is,  as  a  rule,  of  German  silver,  the 
specific  resistance  of  which  metal  is  but  little  affected  by 
variations  of  temperature.  The  wire  is  usually  insulated 
by  two  coverings  of  silk  and  is  wound  double  on  ebonite 
bobbins,  the  object  of  the  double  winding  being  to  elimi- 
nate the  extra  current  which  would  be  induced  in  the  coils 
if  the  wire  were  wound  on  single.  By  double  winding  the 

*  "A  Hand  Book  of  Electrical  Testing,"  by  H.  R.  Kempe.  Fifth 
edition  London:  E.  &  N.  Spon.  1892.  578  pages,  200  Illustrations. 
Price.  «7.25. 


84  ELECTRICAL  MEASUREMENTS. 

current  flows  in  two  opposite  directions  on  the  bobbin,  the 
portion  in  one  direction  eliminating  the  inductive  effect  of 
the  portion  in  the  other  direction.  When  wound,  the  bob 
bins  are  saturated  in  hot  paraffine  wax,  which  thoroughly 
preserves  their  insulation,  nnd  prevents  the  silk  covering 
from  becoming  damp,  which  would  have  the  effect  of  par- 
tially short-circuiting  the  coils  and  thereby  reducing  their 
resistance. 

The  small  resistances  are  made  of  thick  wire,  the  higher 
ones  of  thin  wire  to  economise  space 

When  bulk  and  weight  are  of  no  consequence,  it  is  better 
to  have  all  the  coils  made  of  thick  wire,  more  especially  if 
high  battery  power  is  used  in  testing,  as  there  is  less 
liability  of  the  coils  to  become  heated  by  the  passage  of 
the  current  through  them. 


IV.—  VOLTAIC  CELLS. 


When  Luigi  Galvani,  in  1786,  first  announced  his 
classic  experiment  with  the  frog's  legs,  it  was 
generally  believed  that  he  had  discovered  the  cause 
of  vitality.  Alexander  Volta,  like  most  of  the  scien- 
tific men  of  his  time,  adopted  this  view,  until  a 
more  careful  study  led  him  to  see  that  what  actu- 
ally caused  the  convulsive  movements  of  the  frog's 
legs  was  electricity,  and  that  Galvani  had  discovered, 
not  the  cause  of  vitality,  but  a  new  method  of  pro- 
ducing electricity. 

Volta,  repeating  the  experiments  of  Galvani, 
found  that  the  movements  of  the  frog's  legs  were 
more  pronounced  when  the  muscles  were  brought 
into  contact  with  the  nerves  by  means  of  two  dis- 
similar metals,  such  as  zinc  and  copper  connected  at 
one  pair  of  ends  and  brought  into  contact  with  the 
nerves  and  muscles  at  the  other  pair  of  ends,  as 
shown  in  Fig.  43. 

As  the  result  of  extended  experiments  in  1800, 

Volta  conceived  of  an  apparatus  for  the  production  of 
(85) 


86  ELECTRICAL  MEASUREMENTS. 

electricity,  which,    in  honor  of  its   inventor,  was 
named  the  voltaic  pile. 

Volta  ascribed  the  origin  of  the  electricity  in  the 
pile,  as  well  as  in  the  experiment  with  the  frog's 
legs,  to  the  contact  of  dissimilar  metals ;  and, 
although  this  view  is  still  held  by  some,  it  is  now 
generally  believed  that  while  the  mere  contact  of 


FIG  43.— GALVANOSCOPIC  FROG. 

dissimilar  metals  will  produce  differences  of 
potential,  it  is  to  the  gradual  oxidization  of  one 
of  the  metals,  in  this  case  the  zinc,  that  that  energy 
is  liberated  which  maintains  an  electric  current  as 
long  as  any  chemical  action  continues. 

A  form  of  voltaic  pile,  similar  to  Volta's  original 
pile,  is  shown  in  Fig.  44,     It  consists  of  alternate 


VOLTAIC  CELLS. 


87 


discs  of  copper,  zinc  and  wet  cloth  placed  in  a  verti- 
cal pile  one  over  the  other.  The  top  and  bottom  of 
the  pile  is  formed  of  a  plate  of  copper  and  a  plate  of 


FIG,  44.— VOLTAIC  PILE. 


zinc  respectively,  which  form  the  poles  of  the  bat- 
tery. 

Any  combination  of  parts  by  means  of  which  elec- 
tricity can  be  produced  in  this  manner  by  chemical 


88  ELECTRICAL  MEASUREMENTS, 

action  is  called  a  voltaic  cell.  The  voltaic  cell  is  by 
some  called  the  galvanic  cell.  When,  however,  it  is 
remembered  that  Galvani  originally  claimed  the  dis- 
covery of  a  vital  fluid,  or  principle  of  life,  and  that 
it  was  Volta  who  first  pointed  out  the  true  cause  of 
the  electricity  produced,  it  will  be  seen  that  the  term 
galvanic  cell  is  a  misnomer. 

A  voltaic  cell  consists  of  two  parts;  namely, 

(1.)  Of  a  voltaic  pair  or  couple. 

(2.)  Of  a  liquid  called  the  electrolyte,  in  which  the 
voltaic  couple  is  immersed. 

The  voltaic  couple  or  pair  generally  consists  of  two 
dissimilar  metals.  It  may,  however,  consist  of 
couples  or  pairs  formed  of  a  great  variety  of  different 
substances;  such,  for  example,  as  metals  and  metal- 
loids, different  gases,  different  liquids,  or  of  differ- 
ent liquids  and  gases.  • 

The  electrolyte  is  generally,  but  not  always,  an 
acid  solution.  It  must  be  capable  of  acting  on  one 
of  the  metals  of  the  voltaic  couple,  arid  of  conducting 
and  of  being  decomposed  by  electricity. 

If  a  plate  of  zinc  and  a  plate  of  copper  be  placed 
in  a  dilute  solution  of  sulphuric  acid  and  water,  and 
left  unconnected  outside  of  the  liquid,  then  the  fol- 
lowing phenomena  take  place  : 

(1.)  The  sulphuric  acid  is  decomposed,  a  salt  of 


VOLTAIC  CELLS.  89 

zinc  called  zinc  sulphate  is  formed,  and  hydrogen 
gas  is  liberated. 

(2.)  The  hydrogen  is  liberated  mainly  at  the  sur- 
face of  the  zinc  plate. 

(3.)  The  entire  mass  of  the  liquid  becomes 
heated. 

If,  however,  the  plates  are  connected  outside  of 
the  battery  by  a  conductor,  then  the  sulphuric  acid 
is  decomposed  as  before,  but  the  remaining  phenom- 
ena are  changed  ;  for, 

(1.)  The  hydrogen  is  now  liberated  mainly  at  the 
surface  of  the  copper  plate. 

(2.)  The  heat  does  not  appear  in  the  liquid  only, 
but  is  distributed  throughout  all  parts  of  the  circuit. 

(3.)  An  electric  current  is  now  produced  which 
flows  through  the  entire  circuit,  and  will  continue 
to  so  flow  as  long  as  any  sulphuric  acid  remains  to 
be  decomposed,  and  zinc  to  unite  with  the  acid  ; 
that  is,  as  long  as  any  chemical  action  takes  place. 

Very  clearly,  then,  the  energy  which  formerly  ap- 
peared as  heat  in  the  liquid  now  appears  in  all 
parts  of  the  electric  circuit  as  electric  energy. 

We  may,  therefore,  regard  the  true  cause  of  the 
production  of  electricity  in  the  voltaic  cell  as  due  to 
the  combination  of  the  zinc  with  the  sulphuric  acid, 
and  not  to  the  contact  of  two  dissimilar  metals. 


QO  ELECTRICAL  MEASUREMENTS. 

In  any  voltaic  couple  consisting  of  two  dissimilar 
metals,  one  of  the  metals  is  acted  on  by  the  electro- 
lyte while  the  other  is  left  untouched.  In  the  case 
of  the  zinc-copper  couple  immersed  in  sulphuric 
acid,  the  metal  which  is  acted  on  by  the  liquid  will 
be  the  zinc. 

When  a  zinc-copper  voltaic  couple  is  immersed  in 
sulphuric  acid,  and  the  circuit  is  completed  outside 
the  battery  by  a  conductor,  the  electric  current  pro- 


FIG.  45.— VOLTAIC  COUPLE. 

duced  will  flow  through  the  liquid  from  the  zinc 
plate  to  the  copperplate,  and  through  the  circuit  ex- 
ternal to  the  liquid,  from  the  copper  plate  to  the  zinc 
plate,  re-entering  the  cell  at  the  zinc  plate. 

In  accordance  with  the  convention  already  made, 
namely,  that  an  electric  current  flows  out  of  a  source 
at  its  positive  pole  and  re-enters  it  at  its  negative  pole, 
it  will  be  seen  that,  in  such  a  couple  as  that  shown  in 
Fig.  45,  the  part  of  the  copper  plate  which  is  out- 


VOLTAIC  CELLS.  91 

side  the  liquid  will  form  the  positive  pole  of  the  cell, 
and  that  the  similar  part  of  the  zinc  plate  will  form 
the  negative  pole.  By  the  same  reasoning,  however, 
that  portion  of  the  zinc  plate  which  is  covered  by  the 
liquid  will  have  a  current  of  electricity  flowing  out 
of  it,  and  will,  therefore,  form  the  positive  pole, 
while  the  similar  portion  of  the  copper  plate  will 
form  the  negative  pole. 

In  other  words,  the  parts  of  the  metallic  plates 
that  are  covered  by  the  electrolyte  are  to  be  regarded 
as  of  different  polarity  from  the  parts  which  project 
from  the  liquid. 

In  point  of  fact  the  zinc  plate  is  all  of  the  same 
polarity,  namely,  negative,  as  an  electrometer  at- 
tached to  this  plate  would  show.  The  apparent  posi- 
tive polarity  of  the  plate  under  the  liquid  is  probably 
due  to  the  polarity  of  that  part  of  the  electrolyte 
which  touches  the  zinc  plate. 

The  ease  with  which  different  metals  form  voltaic 
couples  with  zinc  renders  it  necessary  to  obtain  plates 
of  chemically  pure  zinc  for  use  in  voltaic  cells,  since 
otherwise  minute  voltaic  couples  will  be  formed 
by  the  particles  of  the  impurities  present,  and  the 
strength  of  the  cell  will  be  wasted  in  producing  cur- 
rents in  minute  closed  circuits.  As  it  is  practically 
impossible  to  obtain  plates  of  chemically  pure  zinc  it 


92  ELECTRICAL  MEASUREMENTS. 

is  necessary  to  cover  the  surface  of  the  zinc  plates 
with  an  amalgam  of  zinc  and  mercury.  This  pro- 
cess of  covering  the  zinc  plate  is  called  the  amalga- 
mation of  the  plate,  and  is  easily  obtained  by  dipping 
the  plate  for  a  few  moments  in  dilute  sulphuric  acid 
and  then  rubbing  a  small  quantity  of  mercury  over 
the  surface.  The  mercury  adheres  to  the  surface  of 
the  zinc  plate,  covering  it  with  a  bright  coating  of 
amalgam. 

During  the  action  of  the  electrolyte  on  the  posi- 
tive plate  of  a  voltaic  couple  the  hydrogen  tends  to 
be  liberated  at  the  surface  of  the  negative  plate,  and 
this  plate  will  finally  become  coated  with  a  cover- 
ing of  hydrogen,  unless  some  means  are  taken  to 
avoid  it. 

It  is  very  important  to  understand  the  effect 
which  this  film  of  hydrogeti  produces  on  a  voltaic 
cell.  It  will  be  remembered  that  the  current  gener- 
ated in  the  voltaic  cell  flows  from  the  positive  plate 
through  the  liquid  to  the  negative  plate.  But  hy- 
drogen is  more  positive  than  zinc,  and  tends,  there- 
fore, to  produce  an  electric  current  in  the  opposite 
direction  to  that  produced  by  the  zinc  ;  namely,  from 
the  copper  plate  to  the  zinc  plate. 

The  hydrogen  gas  does  not  actually  produce  this 
current ;  it  only  te-nds  to  produce  it ;  or,  in  other 


VOLTAIC  CELLS.  93 

words,  the  hydrogen  produces  what  is  called  a  coun- 
ter-electromotive force,  and  the  cell  undergoes  what 
is  culled  polarization.  The  polarization  of  a  voltaic 
cell  causes  a  weakening  of  the  current  produced,  for 
the  following  reasons  : 

(1.)  On  account  of  the  counter-electromotive  force 
produced,  which  causes  a  decrease  in  the  effective 
electromotive  force  of  the  cell. 

(2.)  On  account  of  the  increased  resistance  of  the 
voltaic  cell,  due  to  the  collection  on  the  surface  of 
the  plate  of  bubbles  of  gas,  which  possess  a  high  re- 
sistance. 

There  are  three  ways  in  which  the  ill  effects  of 
polarization  may  be  avoided,  namely  : 

(1.)  Mechanically.  The  bubbles  of  gas  are  brushed 
off  the  surface  of  the  negative  plate  by  means  of  a 
stream  of  air  or  of  liquid.  Or,  they  are  permitted 
to  pass  off  by  roughening  the  surface  of  the  plate 
and  thus  covering  it  with  points. 

(2.)  Chemically.  The  surface  of  the  negative 
plate  is  surrounded  by  some  powerful  oxidizing  sub- 
stance like  nitric  or  chromic  acid,  which  is  capable 
of  oxidizing  the  hydrogen  and  thus  removing  it  from 
the  plate. 

(3.)  Electro-chemically.  The  negative  plate  is  im- 
mersed in  a  solution  of  the  same  metal  as  that  of 


94  ELECTRICAL  MEASUREMENTS. 

which  it  is  composed.  For  example,  if  copper 
forms  the  negative  plate  of  the  cell,  it  is  immersed  in 
a  solution  of  copper  sulphate,  so  that  the  hydrogen, 
which  tends  to  be  liberated  at  the  surface  of  the  cop- 
per plate,  in  passing  through  the  solution  of  the  cop- 
per salt  surrounding  this  plate,  decomposes  the  salt 
and  deposits  a  film  of  copper  over  the  plate. 

Although  there  are  a  great  variety  of  the  voltaic 


FIG.  46.— SMEE  CELL. 

cells,  yet  they  may  readily  be  divided  into  two  great 
classes ;  namely, 

(1.)  Single-fluid  cells. 

(2.)  Double-fluid  cells. 

In  single-fluid  cells,  as  the  name  implies,  the  vol- 
taic couple  or  pair  is  dipped  into  a  single  electrolytic 
fluid,  while  in  the  double-fluid  cell  each  element  or 
plate  of  the  couple  is  dipped  into  a  different  fluid. 


VOLTAIC  CELLS.  95 

As  a  rule,  double-fluid  cells  are  less  liable  to  polariza- 
tion and  are  capable  of  giving  a  constant  current  for 
a  longer  time  than  single-fluid  cells. 

The  well-known  form  of  single-fluid  cell  shown  in 
Fig.  46  is  called,  after  its  inventor,  the  Smee  cell. 
The  voltaic  couple  or  pair  is  formed  of  zinc  and  sil- 
ver. The  silver  plate  is  generally  placed  between 


FIG.  47.— BICHROMATE  CELL. 

two  plates  of  zinc.     The  electrolyte   employed  is 
dilute  sulphuric  acid. 

The  silver  plate  has  its  surface  roughened  by  a 
covering  or  coating  of  platinum  in  a  finely  divided 
state  known  as  platinum  black.  This  cell  produces 
an  electromotive  force  of  about  .65  volt. 


96 


ELECTRICAL  MEASUREMENTS. 


Another  well-known  form  of  single-fluid  cell, 
shown  in  Fig.  47,  is  known  as  the  bichromate  cell. 
The  voltaic  couple  consists  of  zinc  and  carbon.  The 
electrolyte  is  formed  by  dissolving  one  pound  of 
bichromate  of  potash  in  ten  pounds  of  water  and 
gradually  adding  to  the  solution  two  and  one-half 


FIG.  48. -GROVE'S  NITRIC  ACID  CELL. 

pounds  of  ordinary  commercial  sulphuric  acid.  The 
electromotive  force  of  this  cell  is  about  1.9  volts. 
The  cell  is  capable  of  giving  a  strong  current ;  but, 
since  it  readily  polarizes,  it  only  furnishes  a  constant 
current  for  a  comparatively  short  time. 

In  double-fluid  cells,  since  there  are  two  separate 


VOLTAIC  CELLS. 


97 


liquids  used  as  electrolytes,  some  means  must  be 
adopted  for  keeping  these  liquids  separate.  This  is 
generally  accomplished  by  the  use  of  a  jar  of  unglazed 
earthenware,  called  a  porous  jar  or  cell. 

Grove's  nitric  acid  cell,  shown  in  Fig.  48,  is  a 
well-known  form  of  double-fluid  cell.  The  \oltaic 
couple  is  formed  by  plates  of  zinc  and  platinum. 
The  platinum  is  placed  in  a  porous  jar  containing 


FIG.  49.— BUNSEN  CELL 

nitric  acid,  and  the  zinc  in  a  cell  containing  dilute 
sulphuric  acid.  The  nitrous  fumes  which  this  cell 
gives  off  during  action  render  its  use  unpleasant. 
It  gives  an  electromotive  force  of  about  1.93  volts. 

The  Bunsen  cell,  shown  in  Fig.  49,  is  a  modifi- 
cation of  the  Grove  cell,  in  which  the  platinum  is  re- 
placed by  a  plate  of  carbon.  As  in  the  Grove  cell, 


98  ELECTRICAL  MEASUREMENTS. 

the  zinc  is  dipped  into  dilute  sulphuric  acid  and  the 
carbon  in  strong  nitric  acid.  This  cell  gives  an 
electromotive  force  of  about  1.96  volts. 

The  great  objection  to  all  double-fluid  cells,  thus 
far  described,  is  to  be  found  in  the  fact  that  the  cur- 
rent which  they  supply  is  far  from  constant.  As  the 
chemical  action  goes  on,  the  strength  of  the  acid 


FIG.  50.— DANIELL'S  CONSTANT  CELL. 

electrolytes  greatly  decreases,  and  a  corresponding 
decrease  occurs  in  the  current  strength.  The  prob- 
lem of  obtaining  a  constant  electric  current  from  a 
voltaic  cell  was  solved  by  Prof.  Daniell,  who  pro- 
duced the  cell  named  after  him,  and  shown  in 
Fig.  50. 


VOLTAIC  CELLS.  99 

In  Daniell's  constant  cell  the  voltaic  couple  is 
formed  of  zinc  and  copper  dipped  respectively  into 
dilute  solutions  of  sulphuric  acid"  and  a  saturated 
solution  of  copper  sulphate  or  bluestone. 

The  sulphuric  acid  is  placed  in  the  outer  cell  and 
the  copper  sulphate  inside  the  porous  cell.  A  per- 
forated cage  or  vessel,  kept  filled  with  crystals  of 
bluestone,  is  supported  so  as  to  have  the  lower 
portions  of  the  crystals  continually  in  contact  with 
the  liquid. 

The  action  on  which  the  constancy  of  the  Daniell 
cell  depends  is  as  follows  :  As  the  sulphuric  acid  of 
the  electrolyte  enters  into  combination  with  the 
zinc,  the  hydrogen,  which  is  thereby  set  free,  passes 
through  the  porous  jar,  but  before  it  reaches  the 
surface  of  the  copper  plate  it  meets  the  solution  of 
copper  sulphate  surrouflding  this  plate,  and,  decom- 
posing it,  deposits  metallic  copper  on  its  surface  and 
sets  free  or  liberates  sulphuric  acid,  which  passes 
through  the  pores  of  the  porous  cup  into  the  outer 
jar.  As  the  strength  of  the  solution  of  copper  sul- 
phate decreases,  from  its  gradual  decomposition, 
enough  crystals  of  the  salt  are  dissolved  from  the 
cage  in  the  upper  part  of  the  liquid  to  keep  the 
solution  saturated,  The  invention  by  Daniell  of  this 
cell  rendered  telegraphy  commercially  possible. 


100  ELECTRICAL  MEASUREMENTS. 

The  Darnell  cell  gives  an  electromotive  force  of 
about  1.072  volts. 

A  serious  objection  to  the  use  of  the  Daniell  cell 
is  to  be  found  in  the  deposit  of  copper  which  is 
formed  on  the  surface  of  the  porous  jar.  This  ob- 
jection has  been  entirely  removed  by  the  invention 
of  a  cell  known  as  the  gravity  cell,  in  which  the  two 


FIG.  51.— THE  GRAVITY  CELL. 

liquids  are   separated  from   each   other  by  means  of 
their  difference  of  density. 

The  gravity  cell  is  shown  in  Fig.  51.  The  copper 
is  made  the  lower  plate,  and  is  kept  covered  by  a 
saturated  solution  of  copper  sulphate,  to  insure 
which  a  large  excess  of  undissolved  crystals  of  copper 
sulphate  are  left  in  the  bottom  of  the  jar  covering 


VOLTAIC  CELLS.  101 

the  copper  plate.  The  zinc  plate  is  suspended  above 
the  copper  plate  in  the  form  of  an  open  wheel,  by 
the  means  shown.  When  in  action  a  sharp  line  of 
demarkation  appears  between  the  denser  blue  solu- 
tion of  copper  sulphate  and  the  less  dense  clear  solu- 
tion of  dilute  sulphuric  acid,  or  hydrogen  sulphate. 
When  zinc  sulphate  is  employed  instead  of  dilute 
sulphuric  acid,  the  electromotive  force  is  somewhat 
lower  than  that  of  the  ordinary  Daniell  cell,  but  the 
constancy  of  the  cell  is  greater. 


FIG.  52.— THK  LKCLANCHE  CELL. 

A  form  of  double-fluid  cell  of  equal  importance 
to  the  Daniell  cell,  called  the  Leclanche  cell,  is 
shown  in  Fig.  52,  where  three  such  cells  are  con- 
nected together  to  form  a  series  battery.  The  voltaic 
couple  is  formed  of  zinc  and  carbon.  The  zinc  is 
immersed  in  an  electrolyte  consisting  of  a  dilute 
solution  of  sal-ammoniac,  while  the  carbon  is  sur- 
rounded by  black  oxide  of  manganese  in  a  finely 
divided  state. 


102  ELECTRICAL  MEASUREMENTS. 

The  carbon  element  consists  of  a  plate  or  rod  of 
carbon  placed  inside  a  porous  cell,  which  contains  a 
mixture  of  broken  gas  retort  carbon  and  finely  divid- 
ed black  oxide  of  manganese.  The  black  oxide  of 
manganese  here  takes  the  place  of  the  other  liquid  in 
the  double-fluid  cell,  since  it  acts  as  the  oxidizing  sub- 
stance which  covers  the  negative  plate,  and  prevents 
it  from  becoming  coated  with  hydrogen  by  oxidizing 
or  removing  such  hydrogen.  The  Leclanche  cell 
gives  an  electromotive  force  of  about  1.47  volts. 
It  readily  polarizes,  and  is,  therefore,  capable  of 
furnishing  a  constant  current  for  but  .a  compara- 
tively short  time.  It  possesses,  however,  the  power 
of  depolarizing  if  left  on  open  circuit,  and,  for  this 
reason,  is  generally  called  an  open-circuited  battery. 

Of  all  the  voltaic  cells  that  have  been  devised,  two 
only,  namely,  the  gravity  and  the  Leclanche,  have 
survived  in  the  struggle  for  existence,  and  come  ex- 
tensively into  general  use.  The  gravity  cell  is  used 
on  what  are  called  closed-circuited  lines,  and  the 
Leclanche  cell  on  what  are  called  open-circuited 
lines. 

The  gravity  cell  is  suitable  for  all  purposes  that 
require  a  constant  current  for  an  indefinite  duration 
of  time,  such,  for  example,  as  in  most  of  the  systems 
of  telegraphy  operated  in  the  United  States,  while 


VOLTAIC  CELLS.  103 

the  Leclanche  cell  is  suitable  for  such  purposes  as 
require  a  momentary  current  for  ringing  bells,  the 
operation  of  annunciators  or  for  other  similar  work, 
and  are  left  on  open  circuit  most  of  the  time. 

A  form  of  voltaic  cell  convenient  for  some  pur- 
poses is  found  in  what  is  called  the  dry  cell.  Such 
a  cell  is  shown  in  Fig.  53.  The  term  dry  cell  is  a 
misnomer,  since  all  such  cells  are  moistened  with 


FIG.  53.— DRY  CELL. 

solutions  of  electrolytes,  and  are,  therefore,  far  from 
dry.  The  moistening  is  obtained  by  the  use  of 
some  hydroscopic  substance  that  absorbs  moisture 
from  the  air. 

The  mistake  is  very  commonly  made  of  calling  a 
voltaic  cell  a  voltaic  battery.  Strictly  speaking,  a 
voltaic  battery,  like  any  other  form  of  battery,  con- 


104 


ELECTRICAL  MEASUREMENTS. 


sists  of  such  a  combination  of  a  number  of   separate 
cells  as  will  permit  them  to  act  as  a  single  cell. 

A  convenient  form  of  voltaic  battery  is  shown  in 
Fig.  54.    When  it  is  desired  to  obtain  a  current,  the 


FIG.  54. — PLUNGE  BATTERY. 


battery  plates  are  lowered  into  the  acid  solution  in 
the  cups ;  when  the  battery  is  no  longer  required  for 
use,  the  plates  are  raised  from  these  liquids. 


VOLTAIC  CELLS.  105 


EXTRACTS  FROM  STANDARD  WORKS. 

Larden,  in  his  "Electricity  for  Public  Schools  and 
Colleges/'*  page  170,  gives  the  following  state- 
ment as  to  the  views  held  by  the  advocates  of  the 
contact  and  of  the  chemical  theory  of  the  origin  of 
the  difference  of  potential  in  the  voltaic  cell  : 

Volta  fixed  his  attention  mainly  on  the  A^  ^difference  of 
potential)  that,  as  it  seemed,  accompanied  the  contact  of 
the  dissimilar  metals  zinc  and  copper.  His  followers  ex- 
aggerated a  certain  one-sidedness  that  existed  in  his  views  ; 
and  the  Contact  school,  as  they  were  called,  considered  that 
the  chemical  solution  of  the  zinc  played  a  subordinate  part 
in  the  action  of  the  cell,  serving  mainly  to  keep  the  sur- 
faces clean  and  so  to  keep  the  same  series  of  bodies  in 
contact.  In  fact  the  word  contact  was  the  keynote  to  their 
theory  of  the  voltaic  cell.  They  considered  the  /\  V  between 
the  terminals  of  the  "  open"  cell  (that  is,  of  a  cell  in  which 
the  terminals  were  insulated)  as  the  algebraic  sum  of  the 
different  A^"s  due  to  the  different  contacts;  of  which,  in 
the  ordinary  Volta's  cell,  the  only  one  of  importance  was 
that  where  the  copper  wire  was  soldered  to  the  zinc. 

The  "Chemical  school"  of  physicists  considered  the  cell 
when  the  circuit  was  closed  and  a  current  was  running. 

•"Electricity  for  Public  Schools  and  ColleRes,"  by  W.  Larden, 
M.A.  London  :  Longmans,  Green  &  Co.  1887.  476  pages,  219 
illustrations.  Price,  31.75. 


106  ELECTRICAL  MEASUREMENTS. 

They  pointed  out  how  the  strength  of  the  current  that 
flowed  was  proportional  to  the  vigour  with  which  the 
chemical  action  proceeded  ;  and  how  the  power  of  the  cell 
depended  on  having  one  plate  as  much  acted  upon,  and  the 
other  plates  as  little  acted  upon,  as  possible. 

Faraday  was  the  great  exponent  of  this  view.  In  modern 
phraseology,  the  "  Chemical  school''  insisted  on  the  chem- 
ical action  as  the  source  of  the  energy  of  the  cell. 

They,  in  their  turn,  were  for  the  most  part  too  one- 
sided ;  and  many  denied  that  dissimilar  metals  in  contact 
did  exhibit  a  difference  of  potential  at  all  without  chemical 
action. 

In  the  next  section  we  shall  attempt  to  show  the  position 
of  modern  theory  and  of  modern  knowledge  in  this  matter  ; 
and  shall  conclude  by  giving  a  view  of  the  Volta's  cell, 
taken  as  a  whole,  which  can  hardly  involve  any  serious 
error. 

But  we  should  add  that  the  whole  question  is  still  to  a 
considerable  extent  unsettled. 


V.— THERMO-ELECTRIC     CELLS    AND 
OTHER  ELECTRIC  SOURCES. 


In  1821  Seebeck,  of  Berlin,  discovered  another 
source  of  electricity  in  the  unequal  heating  of  dis 
similar  metals.  He  found,  when  two  dissimilar 
metals  are  formed  into  a  circuit  by  soldering  their 
junctions  together,  that  when  one  of  the  junctions 
is  heated  above  the  temperature  of  the  other  junc- 
tion a  current  of  electricity  is  produced,  which 
flows  through  the  circuit  in  a  certain  direction,  and 
that,  when  this  junction  is  cooled  below  the  tempera- 
ture of  the  other  junction,  the  current  so  produced 
flows  in  the  opposite  direction. 

He  called  such  currents  thermo-electric  currents 
and  the  electricity  produced  by  them  thermo-elec- 
tricity. 

The  two  metals  or  other  substances  forming  a 
thermo-electric  combination  are  called  a  thermo- 
electric couple,  and  each  of  the  substances  forming 
such  a  couple,  the  thermo-electric  element. 

Thermo-electric  phenomena  also  occur  at  the 
junctions  of  two  dissimilar  liquids,  or  at  the  junc- 
tion of  a  liquid  and  a  metal,  when  such  junctions  are 
unequally  heated. 

(107) 


108  ELECTRICAL  MEASUREMENTS. 

One  of  the  simplest  ways  in  which  the  production 
of  thermo-electric  currents  can  be  shown  is  by  sol- 
dering two  different  metals  together  at  one  end  and 
connecting  their  other  ends  to  the  terminals  of  a 
galvanometer.  When  the  junction  is  either  heated 
or  cooled  a  current  of  electricity  is  produced  which 
deflects  the  needle  of  the  galvanometer.  This  deflec- 
tion occurs  in  one  direction  when  the  junction  is 
heated,  and  in  the  opposite  direction  when  it  is  cooled. 

In  the  following  table  the  different  metals  are  ar- 
ranged in  such  an  order  that  each  metal  acquires 
positive  electricity  when  combined  with  any  metal 
following  it,  and  negative  electricity  when  combined 
with  any  metal  preceding  it.  Or,  in  other  words, 
when  the  junction  of  two  such  metals  is  heated  the 
current  passes  across  the  junction  through  the  solder 
from  the  +,  or  positive  metal,  to  the  — ,  or  negative 
metal.  Such  a  series  of  metals  is  called  a  thermo- 
electric series.  A  thermo-electric  series  is  given  in 
the  following  table  : 


Cobalt  

+9 

Zinc 

0  2 

Potassium  
Nickel 

+5.5 
T  5 

Cadmium  

....  —    0.3 
2  0 

Sodium  

-f  3 

—    38 

Lead  

+  1  03 

5  2 

Tin 

+  1 

9  6 

Copper  , 

—    98 

Silver  

J-  +  1 

Tellurium 

—179  9 

Platinum  

...  +0.7 

Selenium.".... 

...  —290.0 

—Oanot. 


THERMO-ELECTRIC  CELLS-OTHER  SOURCES.    109 

For  example,  in  the  bismuth-antimony  couple, 
when  the  junction  is  heated,  the  current  passes 
from  the  bismuth,  the  positive  metal,  across  the  junc- 
tion to  the  antimony,  the  negative  metal. 

The  meaning  of  the  numbers  is  as  follows  :  calling 
the  electromotive  force  of  a  copper-silver  couple 
unity,  the  electromotive  force  of  any  other  pair 
where  the  signs  are  the  same  is  equal  to  the  differ- 
ence of  the  numbers,  but  where  the  signs  are  differ- 
ent is  equal  to  their  sum.  For  example,  the  elec- 
tromotive force  of  the  bismuth-nickel  couple  is  25  — 
5=20  times  that  of  the  silver-copper  couple  ;  that  of 
the  bismuth-antimony  couple,  25  +  9.8  =  34.8. 

For  small  differences  of  temperature  the  electro- 
motive forces  produced  are  proportional  to  the 
temperature.  As  the  temperature  increases  the 
electromotive  force  decreases  until,  at  a  certain 
temperature  of  the  hot  junction,  called  the  neutral 
temperature,  no  difference  of  potential  is  produced. 

A  thermo-electric  couple  joined  in  a  circuit  so  as 
to  produce  electricity  is  called  a  thermo-electric  cell. 
A  number  of  thermo-  electric  cells,  so  arranged  as  to 
act  as  a  single  source  or  cell,  is  called  a  thermo-elec- 
tric battery. 

The  difference  of  potential  produced  by  any  ther- 
mo-electric couple  is  approximately  proportional  to 


110  ELECTRICAL  MEASUREMENTS. 

the  difference  of  temperature  of  its  junctions,  pro- 
vided such  difference  of  temperature  is  not  too 
great.  The  actual  difference  of  potential  produced 
by  the  best  thermo-electric  couples  is  quite  low.  In 
the  case  of  one  of  the  most  powerful  of  such 
couples,  namely,  that  of  bismuth-antimony,  the 
electromotive  force  or  difference  of  potential  for  one 
degree  centigrade  difference  of  temperature  is  only 
117  micro-volts.  In  order,  therefore,  to  increase 


FIQ.  55— SERIES-CONNECTED  THERMO-ELECTRIC  C00PLE3. 

the  difference  of  potential,  a  number  of  separate 
thermo-electric  cells  are  joined  together  so  as  to 
form  a  thermo-electric  battery. 

The  manner  in  which  the  separate  thermo-electric 
cells  are  connected  in  series  to  form  a  thermo-elec- 
tric-battery is  shown  in  Fig.  55,  which  represents 
a  thermo-electric  battery  known  as  Nobilli's  thermo- 
electric pile,  or  battery,  after  the  name  of  its  in- 
ventor. 


TBERMO-ELECTRIC  CELLS— OTHER  SOURCES.    HI 

Here  a  number  of  bismuth-antimony  couples  are 
carefully  insulated  from  one  another  in  all  parts  of 
their  circuits  except  at  their  junctions,  which  are 
soldered  together  and  are  then  piled  so  as  to  form 
the  cubical  pile,  shown  in  Fig.  5G. 

The  different  couples  in  this  pile  are  connected 
throughout  in  series,  so  that  the  difference  of  poten- 
tial produced  increases  in  the  direct  proportion  of 
the  number  of  separate  cells  connected  together. 


FIQ.  56.— NOBILLI'S  THERMO  ELECTRIC  PILE. 

The  battery,  or  thermo-electric  pile,  is  placed  in  a 
metallic  box,  and  the  free  terminals  of  the  first  and 
last  couples  are  connected  to  the  binding  posts  to 
form  the  terminals  of  the  pile.  These  binding 
posts  are  seen  in  the  figure  at  the  top  of  the  pile. 

If  the  separate  junctions  of  the  thermo-electric 
couples  be  numbered  successively  from  the  first  to 
the  last  it  will  be  seen  that  when  they  are  arranged 


112       .  ELECTRICAL  MEASUREMENTS. 

as  shown  in  Fig.  56,  all  the  even  junctions  are 
situated  at  one  face  of  the  pile  and  all  the  odd 
junctions  at  its  opposite  face.  This  is  necessary  in 
order  that  the  separate  thermo-electric  differences  of 
potential  generated  by  the  separate  couples  shall  be 
added  together  ;  for,  if  a  thermo-electric  circuit  be 
formed  as  shown  in  Fig.  57  and  all  its  junctions 
are  equally  heated,  no  thermo-electric  currents  will 
be  produced,  since  the  differences  of  potential  there- 
by generated  neutralize  one  another.  It  is  not 
actually  necessary  to  employ  different  metals  to  form 


FIG.  57.— THERMO-ELECTRIC  CIRCUIT. 

thermo-electric  couples,  since  couples  formed  of  even 
the  same  metals,  under  different  physical  conditions, 
will  produce  thermo-electric  currents  when  un- 
equally heated.  Such  couples,  however,  are  gen- 
erally very  weak. 

For  example,  if  a  conductor  such  as  a  wire,  a 
part  of  whose  length  is  bent  on  itself,  and  the  re- 
mainder of  which  is  straight,  as  shown  in  Fig.  58, 
be  heated  at  the  straight  part  by  the  flame  F,  of  a 
lamp,  a  difference  of  potential  will  be  produced,  as 


THERMO-ELECTRIC  CELLS-OTHER  SOURCES.    113 

can  be  shown  by  connecting  the  ends  of  the  wire 
with  a  galvanometer. 

Thermo-piles  have  been  constructed  by  Clamond 
of  couples  of  iron  and  an  alloy  of  zinc  and  antimony 
of  sufficient  power  to  sustain  a  voltaic  arc  producing 
a  light  equal  to  40  carcel  burners.  Many  practical 
difficulties  exist,  however,  which  will  have  to  be 
overcome  before  thermo-piles  can  be  commercially 
employed  as  electric  sources. 
F 


FIG.  58.— THERMO-ELECTRICITY. 

It  is,  however,  in  the  production  of  electricity 
directly  from  heat  produced  by  burning  coal  that 
the  greatest  advance  in  the  science  of  electricity  is 
to  be  expected  in  the  near  future,  and  it  is  possibly 
in  the  direction  of  thermo  electric  piles  that  such 
advance  is  to  be  effected. 

In  thermo-electric  piles,  considered  as  sources  of 
electricity,  although  the  contact  of  dissimilar  metals 
is  necessary  for  such  production,  yet  the  source  of 
the  energy  which  must  be  expended  to  maintain  such 


114  ELECTRICAL  MEASUREMENTS. 

current  is  undoubtedly  to  be  found  in  the  heat 
energy,  for  the  heat  applied  at  the  heated  junctions 
disappears  more  rapidly  when  the  circuit  is  closed 
than  when  it  is  opened. 

A  variety  of  electric  cell,  which  depends  for  its 
source  of  energy  on  light,  or  luminous  radiant  energy, 
rather  than  on  heat,  or  non-luminous  radiant  energy, 
is  to  be  found  in  the  photo-electric  cell. 

Photo-electric  cells  are  made  in  a  variety  of  forms 
and  of  a  number  of  different  materials  ;  selenium, 
however,  a  comparatively  rare  substance,  generally 
found  associated  with  sulphur,  is  most  frequently 
employed  for  this  purpose. 

One  of  the  simplest  forms  of  selenium  cell  consists 
of  a  mass  of  selenium  that  has  been  fused  between 
two  conducting  wires  of  platinized  silver  or  other 
suitable  material.  The  platinized  silver  wire  is 
wound  in  two  separate  spirals  around  a  cylinder  of 
hard  wood,  care  being  taken  to  keep  the  two  wires 
a  constant  distance  apart  so  as  to  avoid  any  direct 
contact  between  them. 

The  space  between  these  two  parallel  wires  is  then 
filled  with  fused  selenium  which  is  allowed  to  cool 
gradually. 

This  construction,  which  permits  the  wires  to  act 
as  the  terminals  of  an  extended  plate  of  selenium,  is 


THERMO-ELECTRIC  CELLS-OTHER  SOURCES.     115 

necessary  in  order  to  decrease  the  resistance  of  the 
cell  so  formed,  the  electrical  conductivity  of  selen- 
ium being  very  poor. 

Such  a  cell  forms  what  is  sometimes  called  a  selen- 
ium resistance,  and,  when  its  opposite  faces  are  un- 
equally exposed  to  sunlight,  so  that  one  is  illumined 
and  the  other  is  kept  dark,  a  difference  of  potential 
is  thereby  produced  which  will  result  in  an  electric 
current,  if  the  terminals  of  the  cell  are  connected 
by  means  of  a  conductor. 

As  in  the  case  of  the  thermo-electric  cell  the  cur- 
rent flows  in  one  direction  if  one  face  is  illumined 
more  than  the  other  face,  and  in  the  opposite  direc- 
tion if  this  face  be  illumined  less  than  the  other. 

Exposure  to  sunlight  reduces  the  resistance  of  a 
selenium  cell  to  about  one-half  its  resistance  in  the 
dark,  but  such  change  of  resistance  does  not  remain 
constant  for  a  long  time. 

A  number  of  curious  applications  have  been  made 
of  the  currents  of  electricity  produced  by  selenium 
cells.  The  following  are  examples  : 

(1.)  A  selenium  cell  is  so  placed  in  a  circuit  con- 
taining an  electro-magnet  and  switch  that,  on  one 
of  its  electrodes  being  exposed  to  the  decreased  illu- 
mination of  coming  night,  the  current  produced  au- 
tomatically turns  on  an  electric  lamp,  and,  converse- 


116  ELECTRICAL  MEASUREMENTS. 

ly,  on  the  approach  of  daylight,  and  the  consequent 
illumination  of  the  electrode,  turns  it  off. 

(2.)  A  device  has  been  proposed  whereby  the 
presence  of  a  light — as,  for  example,  that  carried 
by  a  burglar — automatically  rings  an  alarm,  and 
thus  calls  the  attention  of  a  watchman  in  the  build- 
ing. 

A  selenium  cell  is  employed  in  connection  with  a 
variety  of  apparatus  as  a  resistance  that  is  automat- 
ically variable  on  exposure  to  light.  When  inserted 
along  with  suitable  electro-receptive  apparatus  in  the 
circuit  of  an  electric  source — such,  for  example,  as 
a  voltaic  battery — on  the  exposure  of  one  of  its  faces 
to  the  light  its  resistance  decreases  and  thus  per- 
mits the  passage  of  'a  stronger  current  through  the 
circuit  in  which  it  is  placed  and  the  consequent  en- 
ergizing of  the  electro  receptive  apparatus.  The 
photophone,  a  variety  of  telephone,  is  constructed 
on  this  principle. 

A  device  called  the  selenium  eye  is  also  constructed 
on  the  same  principle.  In  this  apparatus  a  dia- 
phragm, the  aperture  of  which  represents  the  pupil 
of  the  eye,  is  automatically  dilated  and  contracted 
by  means  of  light,  which  falls  on  a  selenium  resist- 
ance. 

Such  an  apparatus  is  shown  in  Fig.  59.     A  selen- 


THERMO-ELECTRIC  CELLS-OTHER  SOURCES.    117 

ium  cell  S,  placed  as  shown,  is  provided  with  two 
slides  or  lids  L,  L.  When  these  are  moved  toward 
each  other  the  amount  of  light  falling  on  the  selen- 
ium resistance  is  decreased  ;  when  they  are  moved 
in  the  opposite  direction  such  amount  is  increased. 

This  motion  may  be  obtained  automatically  by 
inserting  in  the  circuit  of  a  voltaic  battery  an  elec- 
tro-magnet, the  movements  of  whose  armature  draw 
the  slots  together,  and  the  movement  of  a  spring 


FIG.  59. -SELENIUM  EYE. 

moves  them  in  opposite  directions  on  the  weakening 
or  cessation  of  the  current.  When,  now,  light  falls 
on  the  selenium  resistance  S,  the  resistance  of  the 
circuit  is  decreased,  and  the  movement  of  the  arma- 
ture of  the  electro-magnet  thereupon  draws  the  slots 
together,  thus  decreasing  the  amount  of  light. 

Van  Uljanin  constructed  a  selenium  cell  as  fol- 
lows :  Selenium  was  melted  in  between  two  parallel 
platinized  silver  wires.  On  being  gradually  cooled 


118  ELECTRICAL  MEASUREMENTS. 

under  pressure,  after  heating  up  to  195°  C.  in  apar- 
affine  bath,  and  this  being  repeated  several  times,  the 
selenium  was  changed  from  the  amorphous  to  the 
sensitive,  crystalline  variety. 

By  the  employment  of  such  a  cell  Van  Uljanin  es- 
tablished the  following  general  principles  : 

(1.)  On  exposure  to  light  an  electromotive  force 
is  developed  which  produces  a  current  flowing  from 
the  dark  or  non-illumined  electrode  to  the  illumined 
electrode. 

(2.)  The  greatest  electromotive  force,  which  is 
0.1/J  volt,  disappears  instantaneously  and  completely 
on  the  removal  of  the  light ;  or,  in  other  words,  the 
action  of  the  light  is  practically  instantaneous. 

(3.)  The  resistance  and  sensitiveness  to  light,  as 
well  as  the  production  of  the  electromotive  force, 
decreases  with  the  age  of  the  cell.  This  is  probably 
due  to  a  gradual  change  in  the  allotropic  state  in  the 
selenium. 

(4.)  The  electromotive  force  is  proportional  to 
the  intensity  of  the  illumination  only  when  the  ob- 
scure rays  or  heat  rays  are  absent. 

Besides  the  photo-electric  cell  already  referred  to, 
differences  of  temperature  in  certain  crystals  are  also 
able  to  produce  electricity.  When  a  crystal  of  tour- 
maline, or  other  pyro-electric  substance,  is  heated  or 


THERMO  ELECTRIC  CELLS-OTHER  SOURCES.    119 


cooled,  it  acquires  electrification  at  points  called  its 
poles. 

In  the  crystal  of  tourmaline  shown  in  Fig.  60  the 
end  A,  called  the  analogous  pole,  acquires  a  posi- 
tive electrification,  and  the  end  B,  called  the  antilo- 
gous pole,  negative  electrification,  while  the  temper- 
ature of  the  crystal  is  rising.  "While  cooling  the 
opposite  electrifications  are  produced. 


FIG.  60.— PYRO-ELKCTRIC  CRYSTAL. 

A  heated  crystal  of  tourmaline,  suspended  by  a  fibre, 
is  attracted  or  repelled  by  an  electrified  body,  or 
by  a  second  heated  tourmaline,  in  the  same  manner 
as  an  electrified  body. 

Many  crystalline  bodies  possess  similar  properties. 
Among  these  are  boracite,  quartz,  tartrate  of  pot- 
ash, sulphate  of  quinine  and  an  ore  of  zinc  known 
for  this  reason  as  electric  calamine. 


120  ELECTRICAL  MEASUREMENTS. 

Another  curious  source  of  electricity  is  to  be  found 
in  the  currents  produced  when  liquids  are  forced  to 
pass  through  the  capillary  spaces  in  thin  walls. 

In  the  form  of  capillary  electrometer  shown  in 
Fig.  61  the  horizontal  glass  tube  B,  filled  with 
mercury,  has  a  drop  of  sulphuric  acid  at  B.  Its 
open  ends  are  connected  with  two  vessels  M  and  N, 
also  filled  with  mercury.  When  an  electric  current 
is  passed  through  the  tube  a  movement  will  be  ob- 
served in  the  drop  of  sulphuric  acid  in  a  direction 


FIG.  61.— CAPILLARY  ELECTROMETER. 

which  changes  with  the  change  in  the  direction  of 
flow  of  current  through  the  tube.  Now  Quinque  has 
shown  that,  if  such  a  drop  of  liquid  be  moved  by 
mechanical  force,  an  electric  current  is  produced 
the  electromotive  force  of  which  will  depend  : 

(1.)  On  the  material  of  the  diaphragm. 

(2.)  On  the  nature  of  the  liquid. 

(3.)  On  the  pressure  required  to  force  the  liquid 
through  the  diaphragm. 


THERMO-ELECTRIC  CELLS-OTHER  SOURCE*.    121 

The  bodies  of  plants  and  animals  are  active 
sources  of  electricity.  The  exact  causes  which  pro- 
duce this  electricity  are  as  yet  unknown,  but  it  is 
certain  that,  in  one  way  or  another,  these  electric 
currents  are  necessary  for  the  carrying  out  of  the 
vital  processes  of  the  animal  or  plant. 

Electricity,  passed  through  the  bodies  of  either 
animals  or  plants,  produces  convulsive  movements 
therein. 

Considerable  advantage  has  been  made  of  such 
electrical  currents  for  the  treatment  of  diseased  con- 
ditions of  the  body.  When  properly  employed,  such 
treatment  is  of  undoubted  value  in  the  curing  of 
certain  diseases,  but  the  use  of  so  powerful  an 
agency  as  electricity  should  not  be  attempted  except 
by  a  skilled  physician. 

Some  animals  appear  to  possess  certain  organs 
especially  designed  to  produce  electric  discharges 
sufficiently  powerful  to  serve  as  a  protection  against 
their  enemies.  Such  is  the  case  with  the  electric 
eel,  a  drawing  of  which  is  shown  in  Fig.  62. 

According  to  Faraday  a  shock  given  by  a  speci- 
men of  electric  eel  which  he  examined  was  equal  to 
that  produced  by  15  Leyden  jars  having  a  total  sur- 
face of  metallic  coating  of  25  square  feet. 

Du  Bois  Raymond  has  shown  that  during  vigorous 


122  ELECTRICAL  MEASUREMENTS. 

growth  plants  are  active  sources  of  electricity.  If 
one  of  the  terminals  of  a  galvanometer  be  inserted  in 
a  fruit  near  its  stem  end,  and  the  other  terminal  in  the 
opposite  end  of  the  fruit,  the  needle  of  the  galvanom- 
eter at  once  shows  the  pressure  of  an  elestric  current. 
Buff  has  shown  that  the  roots  and  the  interior 


FIG.  62.— ELECTRIC  EEL. 

portions  of  plants  are  always  negatively  charged, 
while  the  flowers,  fruits  and  green  twigs  are  posi- 
tively charged. 

Plant-tissue,  or  fibre,  like  the  muscular  fibre  of 
animals,  exhibits  in  many  cases  a  true  contraction 
on  the  passage  through  it  of  an  electric  current. 


THERMO-ELECTRIC  CELLS-OTHER  SOURCES. 


EXTRACTS  FROM  STANDARD  WORKS. 

In  Noad's  "  Student's  Text- Book  of  Electricity,"* 
revised  by  Preece,  the  following  facts  are  given  con- 
cerning some  forms  of  thermo-electric  piles,  p.  396: 

Prof.  Dove  employed  iron  and  platinum  soldered 
together  in  alternate  lengths,  and  the  whole  wound  on  a 
cylinder  of  such  diameter  as  to  bring  all  the  iron-platinum 
junctions  on  one  side  of  the  cylinder,  and  all  the  platinum- 
iron  junctions  on  the  other. 

Farmer,  in  America,  used  Marcus  metal  and  German 
silver,  but  failed  to  secure  a  good  permanent  connection. 

Instead  of  bismuth  and  antimony,  Bunsen  used  as  the 
elements  of  his  thermo-battery  copper  pyrites  combined 
with  copper,  or  pyrolusite  combined  with  copper  or  plat- 
inum. Ten  of  such  combinations  give  all  the  actions  of  a 
Daniell  battery,  having  an  effective  copper  surface  of  14 
square  centimetres  (2.17  square  inches)  in  area.  Stefan 
employed  granulated  sulphide  of  lead  for  the  positive,  and 
copper  pyrites  for  the  negative  element — the  power  of  a 
single  pair,  as  compared  with  a  Daniell's  cell,  is  stated  to 
be  as  1  to  5.5.  Marcus,  in  Germany,  used  for  the  positive 
metal  an  alloy  composed  of  copper,  10  parts  ;  zinc,  6  parts  ; 
nickel,  6  parts  ;  and  for  the  negative  an  alloy  composed  of 
antimony,  1 !  parts  ;  zinc,  5  parts  ;  bismuth,  1  part. 

*  "The  Student's  Text  Book  of  Electricity,"  by  Henry  M.  Noad, 
Ph.  D.,  F.  R.  S.  Revised  by  W.  H.  Preece,  M.  I.  C.  E.  London: 
Crosby  Lockwood  &  Co.  1879.  615  pages,  471  illustrations.  Price  $4, 


134  ELECTRICAL  MEASUREMENTS. 

The  elements  of  his  battery  are  about  7  inches  long,  7 
lines  broad,  and  half  a  line  thick;  they  are  screwed  to- 
gether, and  so  arranged  that  their  lower  junctions  can  be 
heated  by  a  row  of  gas  jets,  and  the  upper  cooled  by  a  cur- 
rent of  water.  The  electromotive  force  of  one  element  is 
equal  to  one-twenty-fifth  of  a  Bunsen's  cell  Six  pairs  de- 
compose water;  thirty  pairs  cause  an  electromagnet  to  lift 
150  pounds;  and  one  hundred  and  twenty-six  pairs  decom- 
pose water  at  the  rate  of  25  cubic  inches  of  mixed  gases 
per  minute,  and  melt  a  platinum  wire  half  a  millimetre  in 
thickness. 

The  conversion  of  heat  into  electricity  by  this  battery,  is 
strikingly  shown  by  the  fact  that  the  water  used  for  cool- 
ing the  uppei  junctions,  is  more  rapidly  vanned  when  the 
current  is  broken ,  than  v,  hen  it  is  closed. 

It  was  stated  by  Marcus,  in  a  communication  to  the 
Austrian  Academy  of  Sciences,  that  he  Lad  constructed  a 
furnace  consuming  240  pounds  of  coal  per  day,  intended  to 
heat  768  elements  of  his  thermo-battery,  the  electromotive 
power  of  which  would  be  equivalent  to  30  cells  of  Bunsen's 
nitric  acid  arrangement. 

Wheatstone  constructed  a  ther mo-pile  on  Marcus's  prin- 
ciple, the  electromotive  force  of  which  was  equal  to  two 
Daniell's  cells ;  he  found  that  its  power  was  greatly  in- 
creased by  repeatedly  melting  the  alloy  composing  the  bars, 
probably  in  consequence  of  their  crystalline  structure  be- 
ing thereby  broken  down. 


VI.— DISTRIBUTION     OF     ELECTRICITY 

BY  DIRECT  OR   CONTINUOUS 

CURRENTS. 


The  electricity  produced  by  any  source  or  battery 
of  sources  is,  by  means  of  variously  arranged  circuits, 
distributed  to  electro-receptive  devices  placed  in 
connection  therewith. 

Any  system  for  the  distribution  of  electricity  con- 
sists, therefore,  of  essentially  the  same  combinations 
of  parts  as  are  found  in  any  circuit  ;  namely, 

(1.)  Of  various  electric  sources  or  batteries  of 
electric  sources. 

(2.)  Of  various  electro- receptive  devices. 

(3.)  Of  conductors  or  leads  connecting  the  electric 
sources,  or  battery  of  electric  sources,  with  the  elec- 
tro-receptive devices. 

The  term  direct  current,  as  generally  employed, 
signifies  a  continuous  current,  or  one  that  flows  con- 
tinuously in  the  same  direction,  as  distinguished 
from  an  alternating  current,  or  one  that  flows  alter- 
nately in  opposite  directions.  The  term  constant 

current  is  sometimes  applied  to  the  case  of  a  cur- 
(125) 


126  ELECTRICAL  MEASUREMENTS. 

rent  that  is  not  only  continuous  in  the  sense  of 
always  flowing  in  the  same  direction,  but  is  also 
such  a  current  as  maintains  approximately  a  con- 
stant current  strength. 

A  number  of  different  systems  have  been  devised 
for  the  distribution  of  electricity  to  the  electro- 
receptive  devices.  Among  the  most  important  of 
these  are  the  following: 

(1.)  Distribution  by  means  of  direct  or  continu- 
ous currents. 

(2.)  Distribution  by  means  of  alternating  currents. 

(3.)  Distribution  by  means  of  storage  batteries,  or 
secondary  generators. 

(4.)  Distribution  by  means  of  condensers. 

(5.)  Distribution  by  means  of    motor-generators. 

The  most  important  purposes  for  which  electricity 
is  distributed  are  for  the  production  of  light,  power, 
heat,  and  for  telegraphic  or  telephonic  communica- 
tion. 

Distribution  by  means  of  direct  or  continuous 
currents  may  be  effected  in  a  variety  of  ways  ;  they 
earn,  however,  be  arranged  under  two  general  classes: 

(1.)  Means  whereby  the  current  is  so  distributed 
over  the  line  wire  or  conductor  that  its  strength 
shall  be  maintained  approximately  constant  notwith- 
standing changes  in  the  number  of  electro-receptive 


DISTRIBUTION  OF  ELECTRICITY.  127 

devices  that  have  been  introduced  into  or  removed 
from  the  circuit.  Such  a  distribution  necessitates 
the  connection  of  the  electro-receptive  devices  to  the 
circuit  in  some  form  of  series  circuit,  and  is,  there- 
fore, generally  called  a  system  of  series  distribution. 
It  is  also  sometimes  called  a  system  of  constant  cur- 
rent distribution. 

(2.)  Means  whereby  electricity  is  so  distributed  over 
line  wires  that  the  potential  difference,  or  electro- 
motive force,  will  be  maintained  approximately  con- 
stant, notwithstanding  changes  in  the  number  of 
electro-receptive  devices  that  have  been  introduced 
into  or  removed  from  the  circuit. 

Such  a  distribution  necessitates  the  connection  of 
the  electro- receptive  devices  to  the  line  in  some  form 
of  multiple  connection,  and  is,  therefore,  generally 
called  a  system  of  multiple-arc  or  parallel  distribu- 
tion. It  is  also  sometimes  called  a  system  of  con- 
stant potential  distribution. 

In  a  system  of  series  distribution,  the  electro- 
receptive  devices  are  connected  to  the  main  line  in 
series  in  the  manner,  shown  in  Fig.  63,  so  that  t'.ie 
current  passes  successively  through  each  of  the 
electro-receptive  devices. 

In  a  series  system  of  distribution,  each  electro- 
receptive  device  added  increases  the  resistance  of  the 


128  ELECTRICAL  MEASUREMENTS. 

circuit,  the  total  resistance  of  the  circuit  being  equal 
to  the  sum  of  the  separate  resistances  placed  therein. 
In  order,  therefore,  to  maintain  the'current  strength 
constant,  the  electromotive  force  of  the  source  must 
be  increased  for  each  electro-receptive  device  added, 
and  must  decrease  for  each  electro-receptive  device 
removed. 

The  number  of  electro-receptive  devices  which  are 
placed  in  series  in  a  series  distribution  circuit  is 
sometimes  so  great  that  the  electromotive  force 
of  the  source  must  be  very  high.  The  high  poten- 


FIG.  63.— SERIES-CONNECTED  ELECTRO-RECEPTIVE  DEVICES. 

tial  required  is  either  obtained  from  a  properly  pro- 
portioned single  source  or  from  a  suitably  connected 
battery  of  separate  sources. 

The  commercial  requirements  of  series  distribu- 
tion necessitate  the  frequent  introduction  and  re- 
moval of  the  electro-receptive  devices  from  the  cir- 
cuit. Means  must,  therefore,  be  devised  whereby 
the  electromotive  force  can  be  automatically  varied 
to  meet  the  requirements  of  the  circuit  at  any  time. 

The  distribution  of  arc  lamps  in  series  forms  one 


DISTRIBUTION  OF  ELECTRICITY.  129 

of  the  principal  purposes  for  which  series  distribu- 
tion is  employed. 

In  systems  of  arc  light  distribution  the  electric 
source  is  almost  invariably  a  dynamo-electric  ma- 
chine, or  battery  of  dynamo-electric  machines.  The 
variations  in  the  electromotive  force  required  to 
maintain  a  constant  current  in  the  circuit,  despite 
changes  in  the  load  or  the  number  of  electro- 
receptive  devices  in  such  circuit,  are  insured  by 
means  of  some  system  of  regulation  whereby  the 
electromotive  force  furnished  by  the  machine  can  be 
altered. 

Such  regulation  may  be  obtained  either  automat- 
ically or  by  means  of  an  attendant. 

The  automatic  regulation  for  a  constant  current 
may  be  obtained  either  by  shifting  the  position  of 
the  collecting  brushes  on  the  commutator  cylinder, 
as  in  the  Thomson-Houston  system,  or  by  the  use  of 
a  variable  resistance  placed  as  a  shunt  to  the  circuit 
of  the  field  magnets  of  the  dynamo,  as  in  the  Brush 
system. 

The  means  employed  in  the  Thomson-Houston 
system  of  automatic  current  regulation  are  shown  in 
connection  Avith  Fig.  64.  The  collecting  brushes 
are  fixed  to  levers  that  are  moved  by  the  regulator 
magnet  R,  the  armature  of  which  is  provided  with 


130 


ELECTRICAL  MEASUREMENTS, 


an  opening  for  the  entrance  of  the  paraboloidal  pole 
piece.  In  order  to  prevent  too  sudden  movements 
of  this  lever  a  dash-pot  is  provided. 

The  adjustment  is  such  that  when  the  current 
strength  is  normal  the  coil  of  the  regulator  magnet 
is  short-circuited  by  contact  points  at  8,  T,  which 
act  as  a  shunt  of  low  resistance.  These  contact 
points  are  opened  or  closed  by  a  solenoidal  magnet 


FIG.  64.— THOMSON-HOUSTON  SYSTEM  OP  AUTOMATIC  REGULATION 

called  the  controller,  whose  coils  are  placed  in  the 
main  circuit. 

The  cores  of  the  solenoidal  magnets  are  suspended 
by  means  of  a  spring.  The  contact  points  are  opened 
when  the  current  becomes  too  strong,  and  the  cur- 
rent traversing  the  coils  of  the  regulator  magnet  A, 
attracts  its  armature,  the  movement  o£  which  shifts 


DISTRIBUTION  OF  ELECTRICITY. 


131 


the  collecting  brushes  into  a  position  on  the  com- 
mutator, at  which  a  smaller  current  is  taken  off. 

In  order  to  decrease  the  spark  that  occurs  at  the 
contact  points  S  and  T,  on  the  opening  of  the  cir- 
cuit, a  carbon  shunt,  r,  of  high  resistance  is  placed 
in  the  circuit  in  the  manner  shown. 

In  actual  operation  the  contact  points  are  contin- 
ually opening  and  closing,  thus  maintaining  a  prac- 
tically constant  current  in  the  series  circuit. 


FIG.  65.— BRUSH  SYSTEM  OP  AUTOMATIC  REGULATION. 

The  system  of  automatic  regulation  employed  by 
Brush  is  shown  in  Fig.  65.  In  this  system  of  reg- 
ulation a  resistance  C,  placed  so  as  to  form  part  of  a 
shunt  circuit  to  the  field  magnets  of  the  machine 
FMt\uaa  its  value  automatically  varied.  This  re- 
sistance is  formed  of  a  pile  of  carbon  plates,  packed, 
as  shown,  in  a  cylindrical  vessel. 

On  an  increase  of  the  current  strength — such,  for 
example,  as  would  result  from  the  extinguishing  of 


132  ELECTRICAL  MEASUREMENTS 

some  of  the  lamps  in  the  circuit — the  electro-magnet 
B,  placed  in  the  main  circuit,  attracts  its  armature 
A,  and  thus  compressing  the  carbon  plates  in  C, 
lowers  their  resistance.  This  lowering  of  the  resist- 
ance diverts  a  larger  proportion  of  the  current  from 
the  field-magnet  coils  F  M,  and  maintains  the 
current  strength  practically  constant. 

In  some  forms  of  dynamo-electric  machines  the 
regulation  is  effected  by  an  assistant.  This  is  ob- 
jectionable. 

In  the  series  distribution  of  electricity  the  current 
is  passed  successively  through  the  electro-receptive 
devices.  In  order  to  prevent  the  failure  of  any 
single  device  from  opening  or  breaking  the  entire 
circuit  some  form  of  safety  device  must  be  employed. 
Such  devices  generally  consist  of  an  arrangement  by 
means  of  which  a  short  circuit  of  comparatively  low 
resistance  is  automatically  established  past  the  faulty 
device. 

In  arc  light  circuits  these  safety  devices  generally 
consist  of  a  circuit  of  low  resistance  formed  of  heavy 
contact  points  that  are  closed  by  means  of  an  elec- 
tro-magnet placed  in  a  shunt  circuit  of  high  resist- 
ance around  the  electrodes  of  the  lamp.  Such  a  de- 
vice practically  forms  an  automatic  switch.  In 
such  cases  the  lamp  is  not  actually  cut  out  or  re- 


DISTRIBUTION  OF  ELECTRICITY.  133 

moved  from  the  circuit,  but  only  practically  so  cut 
out,  since  most  of  the  current  passes  through  the 
circuit  of  low  resistance. 

In  order  to  readily  cut  out  or  remove  a  lamp  from 
the  line  by  hand,  so,  for  example,  as  to  permit  it 
to  be  safely  recarboned,  a  hand  switch  is  provided 
by  means  of  which  a  by-path  of  low  resistance  is 
closed  around  the  lamp,  thus  practically  cutting  it 
from  the  circuit.  ** 

In  a  system  of  multiple-arc,  or  parallel  distribu- 
tion, the  electro-receptive  devices  are  connected  to 


FIG.  66.— MULTIPLE-CONNECTED  ELECTRO-RECEPTIVE  DEVICES. 

the  leads  or  conductors,  which  constitute  the  main 
line,  in  some  of  the  varieties  of  multiple  or  parallel 
connections.  Each  of  the  devices  added  decreases 
the  resistance  of  the  circuit,  since  it  increases  the 
area  of  cross  section  of  the  conductors  that  connect 
the  opposite  leads. 

The  multiple-arc  connection  of  six  electro-recep- 
tive devices,  I,  2,  3,  4,  5  and  6,  to  the  leads  (7,  C, 
and  C',  C',  is  shown  in  Fig.  66. 


134  ELECTRICAL  MEASUREMENTS. 

In  order  to  maintain  constant  the  strength  of  the 
current  which  passes  through  each  device,  notwith- 
standing a  change  in  the  number  of  such  devices, 
the  difference  of  potential  at  the  terminals  of  each 
of  the  devices  added,  which  are  here  supposed  to  be 
of  the  same  resistance,  must  likewise  be  maintained 
constant. 

It  will  be  remembered  that  in  the  scries  distribu- 
tion circuit  the  current  strength  mtTst  be  maintained 
constant  in  order  to  insure  the  passage  of  the  same 
current  through  each  device  added  to  the  circuit,  no 
matter  how  many  might  be  placed  therein  at  any 
one  time. 

In  the  multiple  connection,  however,  it  is  the  po- 
tential or  the  electromotive  force  of  the  leads  to 
which  the  circuit  is  connected  that  must  be  main- 
tained constant.  The  series-connected  circuit,  as 
already  mentioned,  therefore,  is  sometimes  called  a 
constant-current  circuit,  and  the  multiple-connected 
circuit  is  sometimes  called  the  constant-potential 
circuit. 

In  the  constant-current  circuit  each  device  added 
to  the  line  in  series  increases  its  resistance.  In  or- 
der to  avoid  too  great  a  resistance  in  such  a  circuit 
the  resistance  of  each  device  added  is  generally 
comparatively  small. 


DISTRIB  UTION  OF  ELECTRICITY.  \  35 

In  the  constant-potential  circuit,  on  the  contrary, 
each  device  placed  in  multiple  between  the  leads 
decreases  the  resistance  of  the  circuit.  In  order  to 
avoid  too  great  a  decrease  in  the  resistance  of  such  a 
circuit,  where  the  number  of  receptive  devices  is 
great,  the  resistance  of  each  separate  device  is  gener- 
ally high. 

The  constant  difference  of  potential  required  on 
the  leads  of  a  constant-potential  circuit  where  dyna- 
mos are  employed  as  the  electric  source  is  generally 
obtained  either  by  some  form  of  compound-winding 
or  by  means  of  hand  regulation,  or  by  both. 

An  automatic  regulation  by  means  of  compound- 
winding  of  dynamos  is  particularly  applicable  to 
constant-potential  machines.  By  compound-wind- 
ing, the  magnetizing  effects  of  the  shunt  coils  is 
maintained  approximately  constant,  while  that  of 
the  series  coils  varies  in  proportion  to  the  load  that 
is  on  the  machine. 

In  compound-wound  machines  the  series  coils  are 
sometimes  wound  close  to  the  poles  of  the  machine 
and  the  shunt  coils  nearer  to  the  yoke  of  the  mag- 
nets, though  custom  varies  somewhat  in  this  re- 
spect. 

The  object  of  compound-winding  is  to  render  the 
dynamo  self-regulating  under  changes  in  its  work- 


136  ELECTRICAL  MEASUREMENTS. 

ing  load.  In  the  compound-wound  dynamo,  the 
shunt  coils  are  often,  for  convenience,  superposed 
on  the  series  coils  and  consist  of  a  much  greater 
number  of  convolutions  of  fine  wire  than  is  placed 
in  the  series  coils,  which  are  of  coarse  wire. 

Suppose,  for  example,  the  terminals  of  the  shunt 
coils  of  a  compound- wound  dynamo  are  connected 
to  the  binding  posts  of  the  machine.  When  the 
current  leaves  the  armature  it  has  two  paths,  one 


FIG.  67.— HAND  REGULATION. 

through  the  thick  series  coils  to  the  external  circuit, 
and  the  other  through  the  finer  and  longer  shunt 
coils.  The  resistance  of  the  shunt  coils  is  so  much 
greater  than  that  of  the  armature  that  the  current 
variations  in  the  armature  will  produce  no  apprecia- 
ble effect  on  the  magnetizing  power  of  the  shunt, 
which,  therefore,  acts  as  a  nearly  uniform  exciter  of 
the  field. 


DISTRIB  UTION  OF  ELECTRICITY.  \  37 

The  hand  regulator  shown  in  Fig.  67  is  a  device 
employed  on  some  of  the  Edison  dynamo-electric 
machines.  The  variable  resistance  at  R,  connected 
to  the  machine  and  one  of  the  leads  as  shown,  is 
provided  with  a  lever-switch,  which  is  operated  by 
hand  whenever  the  potential  rises  above  or  falls 
below  its  proper  value.  By  these  means  an  approxi- 
mately constant  potential  is  maintained  on  the  leads  to 
which  the  lamps  L,  L,  L,  are  connected  in  multiple. 

In  systems  of  multiple  connection,  since  the  elec- 
tro-receptive devices  are  connected  to  the  leads  in 
multiple,  the  opening  of  the  circuit  of  any  single 
device  does  not  interfere  with  the  operation  of  the 
remainder  of  the  devices  placed  therein.  It  is  nec- 
essary, however,  in  practice,  for  the  purpose  of  pre- 
venting abnormally  great  currents  from  passing 
through  the  circuit  of  any  single  device,  and  thereby 
destroying  the  balance  of  the  rest  of  the  circuit, 
or  raising  the  temperature  of  the  circuit  danger- 
ously high,  to  employ  devices  called  safety-fuses, 
strips  or  plugs.  These  consist  essentially  of  strips, 
plates  or  bars  of  lead,  or  some  other  readily 
fusible  alloy,  which  are  placed  directly  in  the  circuit, 
and  which  fuse  and  automatically  break  the  circuit 
on  the  passage  of  any  current  that  would  injure  the 
safety  of  other  parts  of  the  circuit. 


138 


ELECTRICAL  MEASUREMENTS. 


These  safety-fuses  are  placed  both  in  the  branch 
circuits  and  in  the  main  Hire  circuits.  Fig.  68  il- 
lustrates such  a  safety-fuse,  as  arranged  for  a  cut- 
out. 


FIG.  68.— CUT-OUT. 

Instead  of  the  distribution  of  incandescent  lamps 
by  multiple  connection  a  multiple-series  connection 
is  often  employed-  because  it  permits  of  a  higher 
difference  of  potential  being  maintained  on  the  leads. 

The  system  of   distribution  known  as  the  three- 


DISTRIBUTION  OF  ELECTRICITY. 


139 


wire  system  is  in  reality  a  modification  of  the  mul- 
tiple-series distribution. 

The  lamps  or  other  electro-receptive  devices  are 
placed  in  multiple-arc  between  either  branch,  and 
are  distributed  so  that  the  current  in  each  branch 
is  approximately  the  same. 

No  current  passes  through  the  central  conductor 
when  the  balance  is  established,  but  when  the 


&_2 

gELD 

^*                     I 

y 

o 

0 
rv 

A. 

T, 

FIG.  69.— THREE-WIRE  SYSTEM. 

balance  is  destroyed  this  central  conductor  takes  up 
such  surplus  current. 

In  the  three-wire  system  double  the  usual  differ- 
ence of  potential  is  used  that  is  required  for  a  single 
lamp,  and  a  considerable  saving  is  thereby  effected 
in  the  cost  of  the  leads  or  mains  used  therewith. 

The   arrangement  of    the   parts  in  a  three-wire 


!40  ELECTRICAL  MEASUREMENTS. 

system  of  distribution  are  shown  in  Fig.  G9.  The 
dynamo  D,  has  its  negative  and  D ',  its  positive  pole 
connected  to  the  conductor  C  C,  called  the  neutral 
conductor.  The  dynamo  D'f  has  its  free  negative 
terminal  connected  to  the  negative  lead  A  A,  and 
the  dynamo  D,  has  its  free  terminal,  the  posi- 
tive terminal,  connected  to  the  positive  lead  B  B. 
The  lamps  are  connected  as  shown  at  L,  L,  L, 
L". 


t 
FIG.  70.— EDISON-HOWKLL  LAMP  INDICATOR. 

An  examination  of  the  drawing  will  show  that 
if  desired  the  entire  difference  of  potential  gen- 
erated by  the  two  dynamos  can  be  fed  to  a  single 
electro-receptive  device  as  shown  at  L",  or  only 
the  difference  of  potential  between  the  neutral  wire 
and  the  other  leads  can  be  utilized,  as  at  L,  L. 

In  any  system  of  incandescent  lamp  distribution 
in  multiple  it  is  necessary  to  know  at  the  central 
station  whether  or  not  the  proper  voltage  or  differ- 


D1STRIB  UTION  OF  ELECTRICITY.  141 

ence  of  potential  exists  on  the  mains.  An  appara- 
tus for  this  purpose,  shown  in  Fig.  70,  is  known  as 
the  Edison-Howell  lamp  indicator. 

The  apparatus  depends  for  its  operation  on  the 
variations  which  are  produced  in  a  carbon  resistance 
consequent  on  changes  in  its  temperature.  The  re- 
sistance employed  for  the  purpose  is  a  carbon  incan- 
descent lamp,  such  as  is  employed  in  the  ordinary 
commercial  circuits.  The  apparatus  consists  essen- 
tially of  a  Wheatstone  bridge  with  resistances  ar- 
ranged as  shown.  A  galvanometer  at  G,  serves  by 
the  movements  of  its  magnetic  needle  as  an  indica- 
tor. Its  needle  remains  at  zero  as  long  as  the  poten- 
tial difference  has  the  exact  voltage  required  on  the 
circuit  with  which  the  indicator  is  connected,  but 
moves  to  one  side  or  the  other  whenever  an  increase 
or  a  decrease  occurs  in  the  potential  difference. 

The  incandescent  lamp  at  L,  which  is  one  of  the 
resistances,  and  is  constantly  traversed  by  the  cur- 
rent, will  have  a  fixed  resistance  for  the  temperature 
at  which  it  is  designed  to  run. 

The  other  resistances  are  so  proportioned  as  to 
insure  the  needle  at  0,  remaining  at  zero.  If, 
however,  the  potential  varies,  the  temperature  of  the 
lamp  L,  varies,  its  resistance  also  varies  and,  being 
carbon,  there  is  a  rise  of  temperature  corresponding 


142  ELECTRICAL  MEASUREMENTS 

to  a  fall  of  lamp  resistance,  which  destroys  the  bal- 
ance of  the  bridge  and  deflects  the  galvanometer 
needle.  The  attendant  then  regulates  the  potential 
to  bring  the  needle  back  to  zero. 

The  necessity  for  an  efficient  lamp  indicator  will 
be  understood  when  it  is  remembered  that  it  is  nee-' 
essary  in  the  commercial  use  of  incandescent  lamps 
to  maintain  as  nearly  a  constant  potential  on  t-he 
mains  as  possible.  A  decrease  in  potential  will  re- 
sult in  a  decrease  in  the  candle  power.  An  increase 
in  potential,  although  attended  by  an  increase  in  the 
candle  power,  produces  a  marked  decrease  in  the 
life  of  the  lamp,  such  decrease  following  an  increase 
of  but  a  few  per  cent,  in  the  difference  of  potential. 


DISTRIBUTION  OF  ELECTRICITY.  143 


EXTRACTS  FROM  STANDARD  WORKS. 

The  following  statements  concerning  current  dis- 
tribution are  made  by  Slingo  and  Brooker  on  page 
579  of  their  "Electrical  Engineering."* 

In  systems  of  distribution  of  electrical  power  by  means 
of  constant  current  the  question  is  comparatively  simple, 
as  the  current  employed  is  not  a  heavy  one,  and  has  the 
same  value  at  all  times  and  in  all  parts  of  the  circuit.  The 
chief  difficulty  likely  to  arise  is  in  providing  for  future 
extensions  of  the  system  when  the  potential  difference 
which  can  be  applied  at  the  ends  of  the  circuit  is  limited. 
The  more  interesting  and  more  difficult  problem  consists  in 
the  supply  of  currents  to  lamps,  or  other  apparatus,  at  a 
constant  potential ;  for  then  the  main  conductors  have  to 
carry  a  very  heavy  and  variable  current.  The  matter 
becomes  more  difficult  if  the  lamps  are  distributed  over  a 
wide  area,  or  are  situated  at  a  distance  from  the  generating 
station .  As  has  been  pointed  out  in  chapter  XIII. ,  the  power 
wasted  may  in  such  cases  be  reduced  to  a  minimum  by  trans- 
mitting it  in  the  form  of  a  small  current  at  high  pressure, 
and  reducing  the  pressure  at  the  required  point  to  a  suitable 
value.  But  such  a  system  has  its  disadvantages.  Although 
the  cost  of  the  copper  is  vastly  reduced,  the  high  potential 
difference  employed  demands  very  efficient  and  expensive 

* "  Electrical  Engineering  for  Electric  Light  Artisans  and 
Students,"  by  W.  Slingo  and  A.  Brooker.  London  :  Longmans, 
Green  &  Co.  1890.  631  pages,  307  illustrations.  Price  |3.50. 


144  ELECTRICAL  MEASUREMENTS. 

insulation,  the  engines  and  dynamos  must  always  be  kept 
running,  and  when  very  little  power  is  being  demanded 
the  efficiency  of  the  transformers  and  the  whole  system  falls 
to  a  low  value.  For  even  when  the  secondary  circuit  of  a 
parallel  transformer  is  disconnected,  some  current  passes 
through  the  primary,  and  when  only  one  or  two  lamps  are 
joined  up,  the  power  appearing  in  the  secondary  may  be  but 
a  comparatively  small  fraction  of  that  absorbed  by  the 
primary.  When  the  number  of  transformers  is  large,  the 
total  power  wasted  becomes  considerable  during  the  time 
when  little  or  no  light  is  required. 

In  the  other  method  of  dist  ributing  direct  from  the  dyna- 
mo to  a  number  of  lamps  all  joined  up  in  parallel,  the  chief 
problems  to  be  faced  are  the  heavy  loss  occurring  in  the 
mains  and  the  difficulty  of  regulating  the  supply  to  each 
lamp. 


VIL—ARC   LIGHTING. 


A  comparatively  short  time  after  the  invention  by 
Volta  of  the  voltaic  pile,  Sir  Humphry  Davy,  by 
means  of  a  powerful  voltaic  pile  of  2,000  couples, 
showed  at  the  Royal  Institution  the  full  splendors 
of  the  voltaic  arc.  Although  this  was  not  the 
first  time  that  light  was  obtained  from  the  car- 
bon arc,  yet  it  was  the  first  time  it  was  publicly 
shown  on  so  extended  a  scale,  and  the  exhibition  prac- 
tically led  to  electric  arc  lighting,  now  so  generally 
employed  for  the  illumination  of  extended  areas. 

When  the  terminals  of  a  sufficiently  powerful 
dynamo  or  other  electric  source  are  connected  to  two 
carbon  pencils  or  rods  that  are  first  placed  in  con- 
tact and  afterward  gradually  separated,  a  brilliant 
arc  or  bow  of  light  appears  between  them.  This  is 
called  the  voltaic  arc,  from  Volta,  the  discoverer  of 
the  battery  by  the  use  of  which  it  was  first  obtained. 
It  takes  the  name  arc  from  its  arc  or  bow  shape. 

In  order  to  form  a  voltaic  arc,  the  carbon  or  other 
electrodes  are  first  placed  together  and  then  gradually 

separated.     The  arc  which  is  formed  between  them 
(145) 


146  ELECTRICAL  MEASUREMENTS. 

consists  mainly  of  volatilized  carbon.  A  part  of  the 
current  raises  the  carbon  to  high  incandescence,  and, 
when  the  carbons  are  gradually  separated,  the  current 
flows  or  passes  from  one  carbon  through  the  mass  of 
glowing  vapor  to  the  other  carbon. 

The  volatilization  of  the  carbon  forms  a  tiny  de- 
pression or  crater  at  the  point  where  the  current 
leaves  one  carbon  and  a  tiny  projection  or  nipple  on 
the  other  carbon,  formed  by  the  deposition  of  that 
portion  of  the  carbon  vapor  which  is  not  consumed 
by  combustion.  This  nipple  consists  of  almost  pure 
graphite. 

When  a  voltaic  arc  is  formed  between  metallic 
electrodes,  a  flaming  arc  is  obtained  the  color  of 
which  is  characteristic  of  the  burning  metal ;  thus 
copper  forms  a  brilliant  green  arc.  The  metallic 
arc,  as  a  rule,  is  much  longer  and  less  brilliant  than 
an  arc  with  the  same  current  taken  between  carbon 
electrodes. 

The  light-giving  power  of  a  heated  body  increases 
very  rapidly  with  its  temperature.  This  increase  in 
light  is  believed  to  be  as  great  as  the  sixth  power  of 
the  increase  in  temperature;  that  is  to  say,  if  the 
temperature  of  any  body  is  doubled,  its  power  of 
emitting  light  will  be  increased  64  times,  or  as 
26.  Although  the  voltaic  arc,  as  well  as  the  car- 


ARC  LIGHTING.  147 

bon  electrodes,  is  intensely  heated,  yet  the  greater 
part  of  the  light  emitted  comes  from  the  tiny  crater 
in  the  positive  carbon.  When,  therefore,  the  vol- 
taic arc  is  employed  as  a  source  of  light,  as  in  the 
arc  lamp,  and  such  light  is  desired  to  be  turned  on 
the  space  below  the  lamp,  care  should  be  taken,  in 
all  forms  of  lamps  where  the  carbons  are  supported 
vertically  one  above  the  other,  that  the  upper  shall 
be  the  positive  carbon. 


FIG.  71.— VOLTAIC  ARC. 

During  the  production  of  the  voltaic  arc  both  pos- 
itive and  negative  carbons  are  consumed  by  gradual 
burning ;  the  positive  carbon,  however,  is  also  con- 
sumed by  volatilization.  The  rate  of  consumption 
of  the  positive  carbon  is,  therefore,  greater  than  that 
of  the  negative  carbon. 


148  ELECTRICAL  MEASUREMENTS. 

The  general  appearance  presented  by  the  carbons 
after  the  voltaic  arc  has  been  established  between 
them  for  some  time  is  shown  in  Fig.  71.  Here  the 
crater  in  the  upper  or  positive  carbon  can  be  seen  at 
its  extreme  end,  as  also  the  nipple  situated  at  the 
opposing  end  of  the  negative  carbon.  The  rounded 
'globules  that  are  seen  OH  the  surface  of  both  carbons 
are  caused  by  deposits  of  molten  matters  which  oc- 
cur as  impurities  in  the  carbons.  In  order  to  employ 
the  voltaic  arc  as  a  source  of  artificial  illumination, 
it  is  necessary  to  maintain  the  carbons  at  a  constant 
distance  apart  during  their  consumption.  This  is 
accomplished  by  means  of  various  devices  called  arc 
lamps,  which  consist  essentially  of  means  by  which 
tlue  carbons  are  automatically  maintained  a  constant 
distance  apart  during  their  consumption. 

The  carbons  may  be  placed  in  various  positions  in 
arc  lights  ;  namely,  either  parallel,  horizontal,  in- 
clined or  vertically  above  one  another.  The  latter 
disposition  is  the  one  generally  adopted. 

An  arc  lamp  consists  essentially  of  the  following 
parts  : 

(1.)  Of  various  feeding  devices  for  maintaining 
the  carbons  at  a  constant  distance  apart  during  their 
consumption. 

(2.)  Of  carbon  holders    for  holding   the   carbon 


ARC  LIGHTING.  149 

pencils,  connected  with  metallic  rods  called  the 
lamp-rods. 

(3.)  Of  various  clutching  or  clamping  devices 
which  grip  or  hold  the  lamp  rod,  and  are  automatic- 
ally released  by  the  action  of  electro-magnets  when 
the  length  of  the  arc  has  exceeded  a  certain  limit. 

In  most  forms  of  lamps  when  the  lamp  is  not  in 
operation  the  carbons  are  in  contact  with  one  an- 
other. On  the  passage  of  the  current,  an  electro- 
magnet through  whose  coils  the  direct  current 
passes,  separates  them  a  short  distance  from  one  an- 
other by  the  movement  of  its  armature,  and  thus 
establishes  an  arc  between  them. 

Various  forms  have  been  given  to  the  feeding  de- 
vices of  arc  lamps.  Where  the  positive  carbon  is 
placed  vertically  above  the  negative  carbon  its 
motion  toward  it  is  accomplished  by  the  action  of 
gravity.  The  lamp-rod  being  held  in  a  fixed  position 
by  the  action  of  a  clutch  or  clamp,  the  carbon  pencil 
connected  with  the  lamp-rod  is  held  at  a  certain  dis- 
tance from  the  fixed  negative  carbon. 

"When,  now,  by  consumption,  the  space  or  interval 
between  the  carbons  becomes  greater,  an  electro-mag- 
net, whose  coils  are  placed  in  a  shunt  circuit  of  high 
resistance  around  the  electrodes,  automatically  re- 
leases the  clutch  or  clamp  when  a  certain  distance 


150  ELECTRICAL  MEASUREMENTS. 

between  the  carbons  has  been  reached,  and  permits 
the  upper  carbon  to  fall  toward  the  lower  carbon. 
In  a  well  constructed  lamp,  however,,  the  carbons 
never  touch  each  other,  the  same  electro-magnetic 
device  which  releases  the-  clamp  automatically 
clamping  it  again  as  soon  as  the  upper  carbon  by  its 
fall  decreases  its  distance  from  the  lower  carbon  by 
the  amount  desired. 

The  automatic  operation  of  this  shunt  magnet 
will  be  understood  from  the  following  considerations: 
The  resistance  of  the  shunt  magnet  coils  is  so  much 
higher  than  the  resistance  of  the  voltaic  arc  that 
when  the  carbons  are  the  proper  distance  apart 
too  small  a  current  strength  flows  in  this  part  of 
the  circuit  to  permit  the  pull  of  the  arma'ture 
of  the  electro-magnet  placed  in  this  circuit  to 
release  the  clutch  and  permit  the  fall  of  the 
lamp-rod.  When,  however,  by  the  consumption 
of  the  carbons,  the  distance  between  them  in- 
creases, and  consequently  the  resistance  of  the  arc 
increases,  a  stronger  current  flows  through  the  coils 
of  the  shunt  magnet,  and,  when  such  lengthening 
of  the  arc  has  reached  a  predetermined  limit,  the 
increased  current,  flowing  through  the  coils  of  the 
shunt  magnet,  becomes  sufficiently  strong  to  release 
the  clutch  or  clamp,  and  thus  permit  the  feeding  of 


ARC  LIGHTING.  151 

the  upper  carbon.  In  a  well  constructed  lamp  such 
feeding  is  almost  imperceptible. 

Arc  lamps  are  generally  placed  in  series  circuits  ; 
that  is,  in  circuits  in  which  the  current  passes  suc- 
cessively through  all  the  lamps  in  the  circuit  and 
returns  to  the  source.  In  order  to  avoid  the  break- 
ing of  the  entire  circuit  through  the  extinguishing 
of  a  single  arc,  an  automatic  safety  device  is  pro- 
vided for  each  lamp,  which  consists  essentially  in  an 
electro-magnet  so  placed  in  a  shunt  circuit  around 
the  arc  that,  as  the  resistance  of  the  arc  becomes 
too  great,  the  increased  current,  which  will  then 
flow  through  the  coils  of  the  magnet,  will  produce  a 
movement  of  its  armature  which  closes  a  short  cir- 
cuit around  the  lamp,  and  thus  cuts  it  out  of  the  cir- 
cuit. 

Arc  lamps  are  made  in  a  variety  of  forms.  A  well- 
known  form  is  shown  in  Pig.  72.  The  space  at 
the  top  of  the  lamp  is  provided  for  the  movement 
of  the  lamp-rod.  The  feeding  mechanism  is  placed 
in  the  cylindrical  box  near  the  top  of  the  lamp 
frame.  The  open  globe  is  placed  around  the  arc  to 
shield  it  from  the  direct  action  of  the  wind,  as  well 
as  for  the  purpose  of  scattering  or  diffusing  the 
light.  For  this  latter  purpose  the  globe  is  generally 
ground. 


152 


ELECTRICAL  MEASUREMENTS. 


A  certain  limit  of  length  of  the  carbon  electrodes 
employed  in  arc  lighting  is  soon  reached  in  actual 
practice  in  all  forms  of  lamps  where  the  electrodes 
are  placed  vertically  one  above  the  other.  Since  the 
lower  end  of  the  upper  carbon  is  placed  near  the 
upper  end  of  the  lower  carbon,  at  the  beginning  of 
the  consumption  the  upper  carbon  and  the  long 


FIG.  72.  —ARC  LAMP. 


lamp-rod  connected  therewith  must  necessarily  ex- 
tend considerably  above  the  point  where  the  arc  ap- 
pears. When,  therefore,  such  a  lamp  is  required 
to  be  hung  near  the  ceiling  of  a  room,  the  maximum 
length  which  can  be  given  to  such  a  rod  is  necessarily 
limited.  It  has  been  found  in  actual  practice  with 


ARC  LIGHTING. 


153 


FIG.  73.— ALL-NIGHT  ARC  LAMP. 

the  length  of  carbon  rods    usually  employed,  that 
when  it  is  desired  to  maintain  the  light  during  the 


154  ELECTRICAL  MEASUREMENTS. 

entire  time  of  darkness,  from  sunset  to  sunrise,  that 
one  pair  of  carbons  will  be  consumed  and  will  re- 
quire to  be  renewed  some  time  during  the  run.  In 
the  early  history  of  arc  lighting  it  was  customary  to 
recarbon  the  lamps,  or  to  replace  the  carbons  during 
the  middle  of  the  night.  A  great  improvement  in 
this  respect  has  been  made  by  the  device  called  the 
double-carbon,  or  all-night  electric  lamp. 

In  the  all-night  electric  lamp,  two  pairs  of  carbon 
rods  are  placed  in  the  lamp,  and  so  connected  with  the 


FIG.  74.— JABLOCHKOFF  CANDLE. 

lamp  circuit  that  when  the  consumption  of  the  first 
pair  has  reached  a  certain  limit,  the  current  is  auto- 
matically shifted  over  to  the  second  pair.  Such  a 
form  of  lamp  is  shown  in  Fig.  73. ' 

In  an  early  form  of  arc  lamp,  called  the  Jabloch- 
koff  candle,  two  candles  placed  parallel  to  each 
other  are  maintained  at  a  constant  distance  apart 
by  some  insulating  material,  such  as  kaolin,  placed 


ARC  LIGHTING.  155 

between  them,  as  shown  in  Fig.  74.  The  current 
passing  into  and  out  of  the  lamp  at  one  end  of  the 
caudle  forms  a  voltaic  arc  at  the  other  end.  As  the 
carbons  are  prevented  from  moving  together  by  the 
insulating  material  placed  between  them,  it  is 
necessary  to  start  the  arc  by  means  of  a  small  strip 
formed  of  a  mixture  oT  some  readily  ignitable  sub- 
stance, called  the  igniter,  placed  between  the  car- 
bons at  the  upper  end  of  the  candle. 

Although  the  arc  starts  at  the  top  of  the  candle 
when  the  carbons  are  of  the  same  length,  it  can  be 
readily  seen  that  the  Jablochkoff  candle  cannot  be 
used  with  a  direct  or  continuous  current,  since,  al- 
though at  the  start  the  carbons  are  of  the  same  length, 
yet  the  more  rapid  consumption  of  the  positive  carbon 
would  soon  cause  its  end  to  fall  so  much  below  the 
level  of  the  negative  carbon  as  to  cause  the  ex- 
tinguishment of  the  light  from  the  too  great  dis- 
tance or  interval  between  them.  For  this  reason 
the  Jablochkoff  candle  is  used  with  alternating  cur- 
rents. 

The  Jablochkoff  candle  was  at  one  time  extensively 
employed  in  arc  lighting,  but  it  has  been  found  in 
practice  that  the  ordinary  arc  lamp  gives  more  eco- 
nomical results.  It  is  a  well-known  fact,  however, 
that  the  Jablochkoff  candle  gives  a  much  more  pleas- 


156  ELECTRICAL  MEASUREMENTS. 

ant  and  steadier  light  than  most  forms  of  arc  lamps. 
The  cause  of  this,  which  is  somewhat  curious,  is  un- 
questionably to  be  found  in  the  fact  that  in  a  Jabloch- 
koff  candle  the  light  is  practically  extinguished  as 
many  times  per  second  as  the  alternating  current 
changes  its  direction.  These  changes  in  direction, 


FIG.  75.— ARC  LAMP  HOOD. 

however,  occur  so  frequently  that  before  the  effect 
produced  on  the  eye  by  one  change  has  passed,  the 
next  effect  follows  it,  and  an  average  effect  is  pro- 
duced which  gives  the  impression  of  a  constant 
light,  the  smaller  changes  of  intensity  that  occur 
during  the  existence  of  any  one  of  these  arcs  being 


ARC  LIGHTING.  157 

so  much  less  than  that  produced  by  the  practically 
total  extinguishment  of  the  arc,  that  the  eye  fails  to 
appreciate  them.  It  will  be  understood,  of  course, 
that,  in  order  to  prevent  the  successive  extinguish- 
ments of  the  arc  from  producing  perceptible  varia- 
tions in  the  intensity  of  the  light,  they  must  follow 
one  another  with  great  rapidity. 

For  the  double  purpose  of  protecting  the  body  of 
the  lamp  from  the  rain  and  aun,  and  for  throwing 


FIQ.  76.— OUTRIGGER  AND  HOOD. 

the  light  downward,  a  conical  shaped  hood  is  pro- 
vided for  all  lamps  exposed  to  the  weather.  Such  a 
hood  is  shown  in  Fig.  75. 

These  hoods  are  placed  either  directly  on  the  top 
of  the  poles  that  hold  the  lamp  and  circuit  wires,  or 
are  suspended  to  the  same  by  suitable  supports. 
When  it  is  desired  to  suspend  such  a  hood  from  the 


158  ELECTRICAL  MEASUREMENTS. 

side  of  a  building,  a  special  form  of  support  called 
an  outrigger  is  provided.  Such  a  form  is  shown  in 
Fig.  7G. 

The  carbon  electrodes  employed  in  the  early  his- 
tory of  arc  lighting  were  made  directly  from  the  de- 
posits of  carbon  that  were  left  in  the  interior  of  the 
gas  retorts  employed  for  the  production  of  illumi- 
nating gas  by  the  destructive  distillation  of  coal. 
They  are  now  made  by  the  following  process  : 

Powdered  coke,  or  gas  retort  carbon,  sometimes 
mixed  with  lamp-black  or  charcoal,  is  made  into  a 
stiff  dough  with  molasses,  tar,  or  some  other  hydro- 
carbon liquid.  The  mixture  is  molded  into  rods, 
pencils,  plates,  bars  or  other  desired  shapes  by  the 
pressure  of  a  powerful  hydraulic  press.  After  drying, 
the  carbons  are  placed  in  crucibles  and  covered  with 
lamp-black  or  powdered  plumbago,  and  raised  to  an 
intense  heat,  at  which  they  are  maintained  for  several 
hours.  By  the  carbonization  of  the  hydro-carbon 
liquid  the  carbon  paste  becomes  strongly  coherent, 
and,  at  the  same  time,  by  the  action  of  heat,  the  con- 
ducting power  of  the  carbon  increases. 

To  give  increased  density  after  baking,  and  so 
prolong  the  life  of  the  pencils,  they  are  sometimes 
soaked  in  a  hydro-carbon  liquid,  and  subjected  to  re- 
baking.  This  may  be  repeated  a  number  of  times. 


ARC  LIGHTING.  159 

Carbons  for  arc  lights  are  generally  copper  coated, 
sp  as  to  insure  a  smaller  resistance,  a  more  uniform 
consumption,  and  a  better  contact  at  the  holders. 

The  unsteadiness  sometimes  noticed  in  arc  lights 
is  due  to  a  variety  of  causes,  the  principal  of  which 
are  the  following  : 

(1.)  Unsteadiness  in  the  driving  power,  either 
arising  from  variations  in  the  amount  of  power,  or 
from  the  slipping  of  the  driving  belt. 

(2.)  Imperfections  in  the  working  of  the  feeding 
mechanism  of  the  lamp. 

(3.)  Impurities  in  the  carbon. 

Unless  the  carbon  electrodes  are  carefully  manu- 
factured they  will  contain  minute  portions  of  materi- 
als that  are  more  readily  volatilized  than  those  form- 
ing the  main  body  of  the  carbon.  When  such  softer 
portions  are  reached,  their  sudden  volatilization 
a  marked  variation  in  the  intensity  of  the  light. 

Much  of  the  unsteadiness  of  the  arc  light  is  due 
to  the  traveling  of  the  arc  from  one  side  of  the  car- 
bon pencil  to  the  other,  so  that  an  observer  on  one 
side  of  the  lamp  will  at  one  moment  see  the  intense 
light  caused  by  the  arc  appearing  on  the  side  nearest 
to  him,  and  at  the  next  moment  will  be  exposed  to  a 
much  less  brilliant  light  by  the  arc  moving  to  the 
side  furthest  from  him.  This  is  especially  the  case 


160 


ELECTRICAL  MEASUREMENTS. 


when  the  carbon  pencils  are  made  too  thick.  It  has 
been  avoided  to  a  certain  extent  in  some  cases  by 
employing  carbon  electrodes  provided  with  a  central 
core  of  charcoal  or  other  softer  carbon,  which  main- 
tains the  arc  centrally  between  the  electrodes. 
•  C 


FIG.  77.— SEMI-INCANDESCENT  LAMP. 
In  the  form  of  lamp  shown  in  Fig.  77  the  source 
of  light  is  due  both  to  the  arc  established  at  J,  be- 
tween the  lower  end  of  the  carbon  and  the  contact 
block  B,  as  well  as  to  the  incandescence  of  the  pen- 
cil or  rod  of  carbon  C,  that  is  maintained  constantly 
in  contact  with  such  block. 


ARC  LIGHTING.  161 


EXTRACTS  FROM  STANDARD   WORKS. 

In  a  work  entitled  "Electric  Light,"*  by  John  W. 
Urquhart,  the  following  description  is  given  of  an 
arc  lamp  on  page  206  : 

When  two  pointed  sticks  of  carbon  attached  to  the  two 
poles  of  a  source  of  electricity,  such  as  any  of  those  previ- 
ously described,  are  touched  together,  a  current  will  pass, 
and  the  carbons  may  then  be  separated  a  certain  distance 
without  interrupting  the  current,  which  is  carried  on  by 
the  intermediate  air  heated  by  the  current,  and  an  exceed- 
ingly brilliant  light,  which  is  termed  the  voltaic  arc,  will 
be  produced  between  the  carbons. 

Particles  of  burning  carbon  are  projected  from  one  car- 
bon to  the  other  and  a  portion  of  the  light  is  attributed 
to  this  flow  of  burning  matter,  but  the  greater  portion  is 
due  to  the  incandescence  of  the  carbon,  or  to  a  conversion 
of  electric  current  into  light,  as  inexplicable  as  that  pro- 
duced in  a  spark  discharged  between  two  conductors,  or 
in  a  flash  of  lightning.  The  researches  of  Captain  Abney, 
R.  E. ,  F.  R.  S. ,  have  shown  that  while  the  white  light  of  the 
positive  pole  is  always  of  the  same  composition  in  respect 
of  the  relative  proportions  of  waves  of  different  colours,  the 
temperature  of  the  arc  from  the  graphite  carbon  is  also  the 

•"Electric  Light,  Its  Production  and  Use."  by  John  W.  Urquhart. 
London:  Crosby  Lock  wood  &  Son.  1891.  407  pages,  145  illustra- 
tions. Price  $3  00. 


162  ELECTRICAL  MEASUREMENTS. 

same  in  arcs  of  different  powers— the  temperature  of  fus- 
ing graphite. 

The  positive  carbon,  or  that  from  which  the  current  is 
generally  assumed  to  flow,  is,  in  voltaic  arc  lamps,  con- 
sumed very  fast,  and  becomes  hollowed  out.  forming  a 
crater,  while  the  negative  or  receiving  carbon  is  acted  upon 
very  slightly,  and  becomes  pointed.  Carbon  rods  may 
burn  at  the  rate  of  about  five  inches  per  hour,  according  to 
their  size,  and  as  they  consume  away  must  be  fed  up  to 
each  other  in  order  to  continue  the  light.  This  was 
formerly  done  by  hand,  but  now  it  is  effected  by  such  per- 
fect automatic  lamps  that  the  light  is  not  only  perfectly 
steady,  but  needs  no  attention  whatever  for  several  hours 
together.  It  is  no  difficult  matter  to  feed  carbons  by  hand, 
by  means  of  a  screw  attached  to  one  of  the  pencils,  and  for 
taking  photographs  by  quick  acting  plates  this  will  answer 
very  well,  but  a  lamp  is  the  only  satis ractory  means  by 
which  ordinary  carbon  rods  can  be  burned  for  general  pur- 


VII I.—  INCANDESCENT  ELECTRIC 
LIGHTING. 


In  the  incandescent  electric  lamp  a  strip  or  fila- 
ment of  carbon,  or  other  refractory  material,  is  heat- 
ed to  incandescence  by  the  passage  through  it  of  an 
electric  current. 

In  order  to  prevent  the  filament,  when  of  carbon, 
from  consuming  or  burning  in  the  air,  it  is  placed 
inside  a  lamp  chamber  from  which  all  the  air  has 
been  exhausted. 

A  substance  suitable  for  use  as  the  incandescing 
conductor  of  an  electric  lamp  should  possess  the  fol- 
lowing properties  : 

(1.)  It  should  be  of  high  refractory  power  ;  that 
is,  it  should  be  capable  of  being  raised  to  a  very  high 
temperature  without  fusing  or  volatilizing. 

(2.)  It  should  possess  a  comparatively  high  elec- 
tric resistance  per  unit  of  length. 

(3.)  It  should  be  electrically  homogeneous  through- 
out all  parts  of  its  length. 

(4.)  It  should  be  capable  of  being  readily  cut  or 
fashioned  into  the  required  shape  before  carboniza- 
tion. 

(163) 


164  ELECTRICAL  MEASUREMENTS. 

Of  the  various  substances  that  have  been  used  for 
the  conductors  of  incandescent  lamps,  carbon,  ob- 
tained by  carbonizing  fibrous  vegetable  material,  ap- 
pears to  most  closely  meet  the  above  requirements. 

Before  being  carbonized  the  fibrous  material  is  cut 
into  the  required  shape  and  then  converted  into  car- 
bon by  any  suitable  carbonizing  process. 

Various  shapes  are  given  to  the  incandescent  fila- 
ment of  the  lamp.  Some  form  of  arc  or  horseshoe 
shape,  however,  «is  generally  employed,  so  as  to  avoid 
the  shadows  which  would  otherwise  be  formed  by 
the  wires  which  carry  the  current  into  and  out  of 
the  lamp  chamber.  In  other  words,  an  arc  shape, 
or  some  modification  of  such  shape,  is  necessary  in 
order  to  permit  the  current  to  enter  and  leave  the- 
lamp  chamber  at  points  near  together,  so  as  to  best 
avoid  shadows,  and  make  it  convenient  to  secure  the 
lamp  .to  a  socket,  which  holds  it  and  contains  the 
contacts  for  the  circuit. 

An  incandescent  electric  lamp  consists  of  the  fol- 
lowing parts,  namely  : 

(1.)  Of  an  incandescing  conductor  or  filament 
through  which  the  current  passes. 

(2.)  Of  an  enclosed  transparent  chamber  called  the 
lamp  chamber. 

(3.)  Of  wires  or  conductors   which  pass  through 


INCANDESCENT  ELECTRIC  LIGHTING.  165 

the  lamp  chamber  and  are  connected  to  the  ends  of 
the  incandescing  filament.  These  are  called  the 
leading-in  wires. 

(4.)  Of  various  devices  for  supporting  the  filament 
inside  the  lamp  chamber  at  its  points  of  connection 
with  the  leading-in  wises. 

(5.)  Of  the  lamp  base,  consisting  generally  of  me- 
tallic points  or  rings  cemented  to  the  base  of  the 


FIG.  78.— LAMP  SOCKET. 

lamp  chamber  and  connected  to  the  leading-in  wires 
which  pass  through  the  lamp  chamber. 

(G.)  Of  the  lamp  socket,  which  consists  of  a  de- 
vice placed  on  the  electrolier  or  bracket  containing 
insulated  contact  plates  or  rings,  connected  to  the 
terminals  of  the  leads  that  furnish  the  lamp  with 
current,  so  that,  when  the  lamp  base  is  merely 


166 


ELECTRICAL  MEASUREMENTS. 


placed  in   the  socket,    the  leading-ill  wires  of  the 
lamp  are  connected  with  tile  leads. 

Two  well-known  forms  of  lamp  sockets  are  shown 
in   Figs.  78  and  79.       Key  switches  are  provided 


FIG.  79.— LAMP  SOCKET. 


for  the  purpose  of  turning  off  the  current  without 
removing  the  lamp  from  the  socket. 

The  lamp  sockets  are    supported    on   brackets, 
pendants    or    electroliers.      Some    forms   of  lamp 


FIG.  80.— LAMP  BRACKETS. 


brackets  are  shown  in  Figs.  80  and  81.     As  in  gas 
fixtures,  the  arms  are  either  fixed  or  movable. 


INCANDESCENT  ELECTRIC  LIGHTING.  167 

As  generally  employed  the  incandescing  filament 
is  produced  as  follows  :  Carefully  selected,  fine- 
grained, bamboo  is  cut  into  strips  of  suitable  length. 
All  the  softer  material  is  removed  from  the  inside  of 
the  strips  as  well  as  the  hard  siliceous  outer  coating. 
In  this  way  a  hard,  fine-grained,  homogeneous  in- 
ner coating  of  fibrous  material  is  obtained,  which 
is  then  cut  by  means  of  planes  or  specially  devised 
cutters  into  strips  of  the  desired  dimensions.  In 
some  forms  of  lamps  the  ends  are  made  of  greater 


FIG.  81.— LAMP  BRACKET,  MOVABLE  ARMS. 

dimensions  in  order  to  insure  good  electrical  connec- 
tion with  the  leading-in  wires. 

This  latter  point  is  of  great  importance  in  the 
proper  operation  of  the  lamp.  As  the  current 
passes  through  the  filament,  it  is  necessary  that  the 
ends  of  the  filament,  where  they  are  placed  in  con- 
nection with  the  leading-in  wires,  should  not  ac- 
quire too  high  a  temperature,  since  otherwise  the 
platinum  of  which  such  wires  are  formed  would  be 
liable  to  fusion.  The  increase  of  temperature  at  this 
point  is  avoided  by  making  the  area  of  cross  sec- 


168  ELECTRICAL  MEASUREMENTS. 

tion  of  the  filament  of  such  junction  larger,  thus 
rendering  its  conducting  power  greater. 

The  bamboo  filament  being  suitably  shaped  is 
now  subjected  to  a  carbonizing  process,  which  is  car- 
ried on  substantially  as  follows :  The  filaments  being 
suitably  shaped  are  bound  or  secured  to  the  outside  of 
a  piece  of  carbon  of  the  shape  it  is  desired  they  shall 
have,  and  are  then  placed  in  boxes,  covered  with 
powdered  plumbago  or  lamp-black,  and  subjected  to 
the  prolonged  action  of  intense  heat  while  out  of 
contact  with  air.  In  this  manner  the  filaments 
maintain  their  shape  during  carbonization. 

The  electrical  conducting  power  of  the  carbon, 
which  results  from  the  carbonizing  process,  is  in- 
creased by  the  action  of  heat,  and  also,  in  all  prob- 
ability, by  the  deposit  throughout  the  mass,  of  carbon 
resulting  from  the  decomposition  of  the  hydro-carbon 
gases  which  are  produced  during  the  carbonization. 

The  carbon  filaments  so  obtained  are  not  yet 
suitable  for  use  in  the  lamp.  No  matter  how 
much  care  has  been  taken  to  select  bamboo 
of  uniform  density,  or  in  cutting  it  into  strips 
of  uniform  area  of  cross  section,  it  will  be 
found,  when  such  strips  or  filaments  are  raised  to 
electric  incandescence  by  the  passage  of  a  current 
through  them,  that  they  do  not  glow  with  equal  brill- 


INCANDESCENT  ELECTRIC  LIGHTING.  169 

iancy  throughout  all  parts  x>f  their  length,  but  cer- 
tain portions  are  much  brighter  than  others.  If,  for 
the  purpose  of  rendering  such  conductors  luminous 
throughout  their  entire  length,  the  strength  of  the 
current  is  increased,  the  "portions  of  high  resistance 
would  either  fuse  or  volatilize,  and  the  filament 
would  be  destroyed ;  or,  if  such  portions  only  were 
allowed  to  give  light,  the  lamp  would  not  be  eco- 
nomical in  its  working. 

In  order  to  avoid  this  difficulty  and  to  render  the 
conductor  fit  for  actual  use  in  the  lamp,  the  filament 
is  subjected  to  a  process  known  technically  as  the 
flashing  process. 

In  the  flashing  process  the  carbon  filament  is 
placed  in  a  vessel  filled  with  the  vapor  of  a  readily 
decomposable  hydro-carbon,  such  as  rhigolene,  and 
gradually  raised  to  electric  incandescence  by  the 
passage  of  a  current.  A  decomposition  of  the 
hydro-carbon  occurs,  the  carbon  resulting  there- 
from being  deposited  both  in  and  on  the  conductor. 

Ordinary  incandescence,  as  by  heat,  would  not 
answer  for  this  process,  since  the  heating  is  not 
properly  directed.  With  electric  incandescence, 
however,  as  the  current  is  gradually  increased  the 
parts  of  the  conductor  where  the  electric  resistance  is 
the  highest  are  first  rendered  incandescent  and  receive 


170  ELECTRICAL  MEASUREMENTS. 

the  deposit  of  carbon.  As  the  current  gradually  in- 
creases, other  portions  become  successively  incan- 
descent and  receive  a  deposit  of  carbon,  until  at  last 
the  filament  glows  with  a  uniform  brilliancy,  indic- 
ative of  its  electric  homogeneity. 

After  the  flashing  process  the  filaments  are  con- 
nected to  the  leadiug-in  wires  and  placed  in  position 
in  the  lamp  chamber.  In  order  to  insure  a  good 
electrical  connection  between  the  ends  of  the  carbon 
filament  and  the  leading-in  conductors  various  de- 
vices are  employed.  One  of  these  consists  in  insert- 
ing the  ends  of  the  carbon  filament  in  cavities  or 
spaces  provided  in  the  conductors,  and  depositing  a 
layer  of  copper  over  the  ends  of  the  wire  and  the 
lower  ends  of  the  carbon  filament  by  the  process  of 
electro-plating. 

Another  process  consists  in  depositing  a  coating 
of  carbon  on  the  joint  by  immersing  the  filament  at 
such  point  in  a  readily  decomposable  hydro-carbon 
liquid,  as,  for  example,  rhigolene,  and  passing  a  suf- 
ficiently powerful  current  through  the  joint  to  raise 
it  to  electrical  incandescence.  The  carbon  resulting 
from  the  decomposition  is  then  deposited  in  a  firmly 
adherent  condition  around  the  joint. 

The  leading-in  wires  are  made  of  platinum,  and 
are  hermetically  sealed  in  the  lamp  chamber  by  being 


INCANDESCENT  ELECTRIC  LIGHTING.  171 

fused  to  the  portions  of  the  glass  through  which 
they  pass.  Platinum  is  employed  for  this  purpose 
because  its  rate  of  expansion  is  so  nearly  the  same  as 
that  of  the  glass  that  it  does  not  injure  the  vacuum 
in  the  lamp  chamber  when  it  alternately  expands 
and  contracts  on  changes  of  temperature. 

The  mounted  carbon  filament  being  placed  inside 
the  lamp  chamber  the  chamber  is  exhausted,  first, 
by  the  action  of  a  mechanical  pump,  by  which  the 
greater  part  of  the  air  is  rapidly  removed,  and 
afterward  by  the  action  of  some  form  of  mercury 
pump. 

The  form  of  pump  frequently  employed  for  this 
latter  purpose  is  known  as  the  Sprengel  mercury 
pump. 

In  this  pump,  as  shown  in  Fig.  82,  the  fall  of  a 
mercury  stream  causes  the  exhaustion  of  a  reservoir 
connected  with  the  vertical  tube,  at  the  point  x,  by 
the  mechanical  action  of  the  falling  mercury  in  en- 
. tangling  bubbles  of  air.  The  floAv  of  the  mercury  can 
be  started  or  stopped  by  means  of  a  clip  stop-cock. 
The  bubbles  of  air  are  largest  at  the  beginning  of  the 
exhaustion,  and  become  smaller  and  smaller  near 
the  end,  until  at  last  the  characteristic  metallic  click 
of  mercury  or  other  liquid  falling  in  a  good  vacuum 
is  heard.  The  exhaustion  may  be  considered  as  com- 


172  ELECTRICAL  MEASUREMENTS. 

pleted  when  the  bubbles  entirely  disappear  from  the 
column. 

In  actual  practice  the  mercury  that  has  fallen  through 
the  tube  is  again  raised  to  the  reservoir  A,  con- 
nected to  the  drop  tube,  by  the  action  of  a  mechan- 
ical pump. 


FIG.  82.-SpRENGEL's  MERCURIAL  AIR  PUMP. 

Care  must  be  taken  to  thoroughly  remove  all  air 
from  the  chamber  of  the  lamp.  Since  the  filament 
is  formed  of  carbon,  if  even  a  small  quantity  is  left 
in  the  chamber  the  life  of  the  lamp,  or  the  time 


INCANDESCENT  ELECTRIC  LIGHTING.  173 

during  which  it  can  continue  to  act  as  an  efficient 
source  of  light,  will  be  greatly  decreased. 

Carbon  possesses  a  marked  power  of  taking  in,  ab- 
sorbing or  occluding  gases,  which  it  condenses  in 
its  pores.  Even  though  the  lamp  chamber  is  ex- 
hausted of  all  the  air  it  contains,  if  it  is  then  her- 
metically sealed  by  the  fusing  of  the  glass,  as  soon 
as  the  carbon  is  raised  to  incandescence  by  the  pas- 
sage of  the  current  through  it,  the  occluded  gas  is 
driven  out  from  the  filament  into  the  lamp  chamber, 
and  soon  destroys  the  filament.  In  order  to  avoid 
this  the  lamp  is  subjected  to  an  operation  known  as 
the  occluded  gas  process. 

This  process  consists  essentially  in  raising  the  fila- 
ment to  incandescence  by  passing  an  electric  current 
through  it  while  the  lamp  is  being  exhausted,  since 
otherwise  the  expelled  gases  would  be  re-absorbed. 
By  this  means  a  considerable  quantity  of  occluded 
gas  is  driven  out  of  the  carbon  which  it  would  be 
impossible  to  get  rid  of  by  the  mere  operation  of 
pumping. 

Both  the  exhaustion  and  the  incandescence  con- 
tinue up  to  the  moment  the  lamp  chamber  is  her- 
metically sealed ;  otherwise  some  of  the  air  might 
remain  in  the  lamp  chamber. 

The  lamp   chamber  is   now   hermetically  sealed, 


174  ELECTRICAL  MEASUREMENTS. 

usually  by  the  fusion  of  the  glass  in  the  manner 
adopted  in  the  sealing  of  Geissler  tubes  or  Crookes' 
radiometers. 

Various  forms  are  given  to  the  incandescent  fila- 
ment. In  the  well-known  form  shown -in  Fig.  83 
the  filament  has  a  horseshoe  shape. 


FIG.  83.— INCANDESCENT  ELECTRIC  LAMP. 

In  the  Swan  lamp,  shown  in  Fig.  84,  the  fila- 
ment is  made  of  cotton  thread  in  the-  form  of  a 
circular  loop.  This  filament  is  made  as  follows  : 
Cotton  threads  are  immersed  for  a  few  moments  in  a 
mixture  consisting  of  two  parts  of  sulphuric  acid 
and  one  of  water,  which  converts  the  cellulose  of  the 
thread  into  artificial  parchment.  As. soon  as  the  fila- 
ments are  removed  from  the  sulphuric  acid  they  are 
rapidly  washed  until  all  traces  of  the  acid  are  re- 
moved. They  are  then  passed  through  dies  so  as  to 
insure  a  uniform  area  of  cross  section,  and  are  wound 


INCANDESCENT  ELECTRIC  LIGHTING. 


175 


on  rods  of  carbon  or  earthenware  of  the  required  out- 
line, packed  in  a  crucible  filled  with  powdered  char- 
coal, so  as  to  exclude  the  air,  and  carbonized. 

Incandescent  lamps  are  generally  connected  to  the 
leads  or  circuits,  either  in  multiple-arc  or  in  multi- 


FiG.84.— SWAN  INCANDESCENT  LAMP. 

pie-series.     They  are;  however,  sometimes  connected 
to  the  line  in  series. 
In  practice  it  is  usual  to  mark  on  incandescent 


176  ELECTRICAL  MEASUREMENTS. 

electric  lamps  the  potential  difference  in  volts  which 
should  be  applied  at  the  terminals  in  order  to 
furnish  the  current  necessary  to  properly  operate 
them.  If  this  potential  difference  is  increased,  the 
light  emitted  increases,  but  the  life  of  the  lamp  is 
shortened. 

When  incandescent  lamps  are  connected  to  the 
leads  in  multiple-arc,  or  in  multiple-series,  which  are 


FIG.  85.— SERIES  INCANDESCENT  ELECTRIC  LAMP. 

the  connections  generally  adopted,  the  resistance  of 
the  filament  is  generally  made  high.  When  they  are 
connected  to  such  line  in  series,  the  resistance  is 
generally  made  low.  The  resistance  of  the  filament 
of  a  series-connected  lamp  is  made  low  because  the 


INCANDESCENT  ELECTRIC  LIGHTING.  177 

cross  sections  of  the  filament  must  be  large  to  carry 
such  large  currents  as  are  generally  employed,  and 
also  because  it  is  more  convenient  in  practice  to 
make  such  filaments  short  for  the  candle  powers 
generally  produced.  A  form  of  series-connected 
lamp  is  shown  in  Fig.  85. 

In  the  case  of  the  series  circuit,  when  any  recep- 
tive device  is  cutout  or  removed  from  the  circuit,  in 
order  to  prevent  the  opening  or  breaking  of  the 
rest  of  the  circuit,  a  path  must  be  provided  by 
which  the  current  can  flow  past  the  device  thus 
removed  or  cut  out.  This  is  usually  accomplished 
by  means  of  a  switch. 

In  a  series-connected  lamp  some  form  of  auto- 
matic switch  or  cut-out  must  be  provided,  which  on 
the  failure  of  any  lamp  in  the  circuit  to  properly 
operate  Avill  automatically  cut  such  defective  lamp 
out  of  the  circuit,  and  will  at  the  same  time  provide 
a  by-circuit  or  path  by  which  the  current  can  flow 
past  the  faulty  lamp  and  feed  the  remaining  lamps 
placed  in  the  circuit. 

This  is  usually  accomplished  by  some  form  of  film 
cut-out  in  which  the  circuit  is  completed  past  the 
faulty  lamp  by  piercing  a  sheet  of  paper  or  mica 
placed  in  a  break  in  the  circuit  between  two  pieces 
of  solder.  On  the  piercing  of  the  film  the  terminals 


178  ELECTRICAL  MEASUREMENTS. 

are  fused  together  and  a  permanent  short  circuit  is 
effected  past  the  lamp. 

In  a  multiple  circuit  the  opening  of  any  lamp 
does  not  affect  the  rest  which  are  still  connected  to 
the  leads.  In  the  multiple-series  circuit  the  open- 
ing of  a  single  lamp  only  affects  the  series  circuit  in 
which  it  is  placed.  Switches,  therefore,  are  not  re- 
quired in  such  circuits. 

The  device  employed  for  automatically  breaking 
the  circuit  when  the  current  has  for  some  reason 


FIG.  86.— SAFETY  FUSK. 

become  dangerously  great  consists  of  some  form 
of  safety  fuse  in  which  a  strip,  plate  or  bar  of  lead, 
or  some  other  readily  fusible  alloy,  that  fuses  and  au- 
tomatically breaks  the  circuit  in  which  it  is  placed  on 
the  passage  of  an  excessive  current  that  would  en- 
danger the  safety  of  other  parts  of  the  circuit. 

Safety  fuses  are  generally  made  of  alloys  of  lead, 
and  are  placed  in  boxes  lined  with   some  non-com- 


INCANDESCENT  ELECTRIC  LIGHTING.  179 

bustible  material  in  order  to  prevent  fires  from  the 
molten  metal. 

Fig.  86  shows  a  fusible  strip  F,  connected  with 
leads  L,  L.  Safety  fuses  are  placed  on  all  branch 
circuits  and  are  made  of  sizes  proportionate  to  the 
safe  carrying  capacity  of  the  circuits  which  they 
guard. 

The  life  of  an  incandescent  electric  lamp  is 
reckoned  by  the  number  of  hours  during  which  it 
can  furnish  an  efficient  source  of  light.  After  being 
used  for  a  time  that  will  depend  on  the  care  with 
which  the  lamp  has  been  constructed,  the  filament 
either  breaks,  or  the  lamp  becomes  useless  from  the 
chamber  gradually  becoming  opaque. 

The  decrease  in  the  transparency  of  the  lamp 
chamber  and  the  consequent  decrease  in  the  efficiency 
of  the  lamp  may  result  either 

(1.)  From  the  settling  of  dust  or  dirt  on  the  outer 
walls  of  the  chamber;  or, 

(2.)  From  the  deposit  of  metal  or  carbon  on  the 
inner  walls  of  the  chamber. 

To  obviate  the  first  cause  of  diminished  transpar- 
ency, the  outside  of  the  lamp  chamber  should  be  fre- 
quently cleaned.  The  diminished  transparency  due 
to  the  second  cause  cannot  be  removed  in  the  lamps 
in  commercial  use,  and  when  it  has  reached  a 


180  ELECTRICAL  MEASUREMENTS. 

certain  point,  it  is  more  economical  to  replace  the 
old  lamp  by  a  new  one. 

In  a  properly  made  lamp  the  dimming  of  the  lamp 
chamber  is  not  apt  to  occur  unless  a  stronger  current 
than  the  normal  current  is  passed  through  the  lamp. 

The  life  of  an  incandescent  lamp  should  not  be 
taken  as  the  time  which  elapses  until  the  filament 
actually  breaks.  As  soon  as  the  lamp  chamber  has 
become  covered  with  such  a  deposit  of  carbon  or 
coating  of  metal  as  to  considerably  decrease  the 


PIG.  87.-PORCELAIN  LAMP  SHADE  AND  WIRE  GUARD. 

amount  of  light  which  passes  through  the  chamber, 
the  lamp  should  be  considered  as  useless. 

The  surface  of  the  lamp  chamber  is  sometimes 
ground  so  as  to  scatter  or  diffuse  the  light.  Reflec- 
tors are  sometimes  placed  back  of  the  lamp  for  the 
purpose  of  throwing  the  light  in  one  general  direc- 
tion. Porcelain  shades  are  often  employed  to  throw 
the  light  downward,  as  shown  in  Fig.  87,  in  which 
is  also  shown  a  wire  shade  guard  provided  for  pro- 
tecting the  shade  in  exposed  situations. 


INCANDESCENT  ELECTRIC  LIGHTING.  181 

EXTRACTS  FROM  STANDARD  WORKS. 

In  the  second  edition  of  his  "  Dictionary  of  Elec- 
trical Words,  Terms  and  Phrases,"*  on  page  274, 
the  author  thus  describes  the  properties  which  should 
be  possessed  by  a  good  artificial  illuminaut  : 

Illumination,  Artificial— The  employment  of  artificial 
sources  of  light. 

A  good  artificial  illuminant  should  possess  the  following 
properties,  namely  : 

(1.)  It  should  give  a  general  or  uniform  illumination  as 
distinguished  from  sharply  marked  regions  of  light  and 
shadow. 

To  this  end  a  number  of  small  lights  well  distributed  are 
preferable  to  a  few  large  lights. 

(2.)  It  should  give  a  steady  light,  uniform  in  brilliancy, 
as  distinguished  from  a  flickering,  unsteady  light.  Sudden 
changes  in  the  intensity  of  a  light  injure  the  eyes  and  pre- 
vent distinct  vision. 

(3.)  It  should  be  economical,  or  not  cost  too  much  to  pro- 
duce. 

(4.)  It  should  be  safe,  or  not  likely  to  cause  loss  of  life  or 
property.  To  this  intent  it  should,  if  possible,  be  inclosod 
in  or  surrounded  by  a  lantern  or  chamber  of  some  incom- 
bustible material,  and  should  preferably  be  lighted  at  a 
distance. 

*  "  A  Dictionary  of  Electrical  Words,  Terms  and  Phrases,"  by 
Edwin  J.  Houston,  A.  M.  Second  edition.  New  York:  The  W,  J. 
Johnston  Co.,  Ltd.  1892.  562  pages,  570  illustrations.  Price  $5.00. 


182  ELECTRICAL  MEASUREMENTS. 

(5.)  It  should  not  give  off  noxious  fumes  or  vapors  when 
in  use,  nor  should  it  unduly  heat  the  air  of  the  space  it 
illumines. 

(6.)  It  should  be  reliable,  or  not  apt  to  be  unexpectedly 
extinguished  when  once  lighted. 

The  electric  incandescent  lamp  is  an  excellent  artificial 
illuminant. 

(1.)  It  is  capable  of  great  subdivision,  and  can,  therefore, 
produce  a  uniform  illumination. 

(2.)  It  is  steady  and  free  from  sudden  changes  in  its  in- 
tensity. 

(3.)  It  compares  favorably  in  point  of  economy  with  coal 
oil  or  gas,  provided  its  extent  of  use  is  sufficiently  great. 

(4.)  It  is  safer  than  any  known  illuminant,  since  it  can 
be  entirely  inclosed,  and  can  be  lighted  from  a  distance 
or  at  the  burner  without  the  dangerous  friction  match. 

The  leads,  however,  must  be  carefully  insulated  and  pro- 
tected by  safety  fuses.  (See  Fuse,  Safety.) 

(5.)  It  gives  off  no  gases,  and  produces  far  less  heat  than 
a  gas-burner  of  the  same  candle-power. 

It  perplexes  many  people  to  understand  why  the  incan- 
descent electric  light  should  not  heat  the  air  of  a  room  as 
much  as  a  gas  light,  since  it  is  quite  as  hot  as  the  gas  light. 
It  must  be  remembered,  however,  that  a  gas-burner,  when 
lighted,  not  only  permits  the  same  quantity  of  gas  to  enter 
the  room  which  would  enter  it  if  the  gas  were  simply 
turned  on  and  not  lighted,  but  that  this  bulk  of  gas  is  still 
given  off,  and  is.  indeed,  considerably  increased  by  the 
combination  of  the  illuminating  gas  with  the  oxygen  of  the 
atmosphere;  and,  moreover,  this  great  bulk  of  gas  escapes 


INCANDESCENT  ELECTRIC  LIGHTING.  183 

as  highly  heated  gases.  Such  gases  are  entirely  absent  in 
the  incandescent  electric  light,  and  consequently  its  power 
of  heating  the  surrounding  air  is  much  less  than  that  of 
gas  lights. 

(6.)  It  is  quite  reliable,  and  will  continue  to  burn  as  long 
as  the  current  is  supplied  to  it. 

Slingo  and  Brooker,  in  a  book  entitled  "  Electrical 
Engineering/'*  on  page  544,  speaks  thus  of  the  in- 
candescent lamp  : 

Although  it  is,  evidently,  a  comparatively  simple 
matter  to  obtain  the  degree  of  exhaustion  necessary  for 
incandescent  lamps,  there  are  several  causes  for  a  deterior- 
ation manifesting  itself  in  the  vacuum  after  the  finished 
lamp  has  been  laid  aside  for  a  time,  such  as  the  occlusion 
of  gases  by  the  carbon  and  platinum,  and  by  the  cement 
employed  to  connect  them  together,  and  the  very  thin  film 
of  air  which  is  liable  to  adhere  to  the  inner  surface  of  the 
bulb.  In  order  to  expel  these  gases,  the  filament  is  raised 
to  incandescence  during  the  later  stages  in  the  process  of 
exhaustion,  or  the  heat  is  applied  externally. 

The  lamp  having  been  sufficiently  exhausted,  the 
small  glass  tube  connecting  the  bulb  to  the  exhaust  tube  is 
fused,  drawn  out  to  a  thread,  and  the  lamp  sealed  off. 

It  remains  now  to  test  its  efficiency,  that  is  to  say,  the 
amount  of  light  emitted  for  a  given  electrical  power.  A 
lamp  may  be  said  to  have  a  very  good  efficiency  if  it  yields 

•"Electrical  Engineering  for  Electric  Light  Artisan,  and  Stu- 
dents," by  W.  Slingo  and  A.  Brooker.  London:  Longmans,  Green 
and  Co.  1890.  631  pages,  307  illustrations.  Price,  $3.50. 


184  ELECTRICAL  MEASUREMENTS. 

one  candle  power  in  return  for  3.5  watts,  so  that  an  average 
16  candle-power  lamp  should  absorb  56  watts. 

The  vacuum  is  usually  tested  by  means  of  an  induction 
coil ;  one  method  is  to  fuse  two  platinum  wires  into  a  glass 
tube  leading  into  the  lamp,  and  simultaneously  exhausted 
with  it,  and  to  connect  these  wires  to  the  terminals  of  the 
secondary  coils.  The  distance  between  the  ends  of  the 
platinum  wires  inside  the  tube  is  so  adjusted  that  when  the 
required  degree  of  exhaustion  is  attained,  the  spark  passes 
through  the  air  outside  the  bulb,  in  preference  to  traversing 
the  vacuous  space  between  tha  platinum  points.  Another 
method  applicable  to  the  finished  lamp  is  to  connect  one 
end  of  the  secondary  to  the  filament,  and  the  other  to  a 
loop  wound  outside  the  bulb,  the  quality  of  the  vacuum 
being  determined  by  the  relative  feebleness  of  the  discharge 
which  takes  place  between  the  filament  and  the  bulb.  It 
should  be  observed  that,  in  a  badly  exhausted  lamp,  not 
only  does  the  filament  "burn,"  that  is,  oxidize,  but  it  also 
requires  a  greater  amount  of  heat  to  raise  and  maintain  its 
temperature  at  the  required  point,  owing  to  the  fact  that 
the  air  particles  carry  a  portion  of  the  heat  away  by  con- 
vection. 


IX.— ALTERNATING  CURRENTS. 


An  alternating  current  of  electricity  is  a  current 
which  flows  alternately  in  opposite  directions,  as  dis- 
tinguished from  a  direct  current  which  flows  contin- 
ually in  one  and  the  same  direction  ;  in  other  words, 
an  alternating  current  is  a  current  whose  direc- 
tion of  flow  is  .continually  reversed,  such  reversals 
rapidly  and  regularly  following  one  another. 

Since  the  current  produced  in  the  armature  of  a 
bi-polar  dynamo-electric  machine  flows  in  one  di- 
rection during  its  rotation  past  one  of  the  poles  of 
the  field  magnets,  and  in  the  opposite  direction  dur- 
ing its  rotation  past  the  other  pole,  the  uncommuted 
currents  from  such  machines  are  alternating  or 
rapidly  reversed  currents. 

In  alternating  currents  the  electromotive  forces 
producing  the  current  are  directed  in  alternately 
opposite  directions.  In  Fig.  88  these  electromotive 
forces  are  represented  in  the  form  of  a  curve,  the 
positive  electromotive  forces,  or  those  which  tend  to 
produce  a  current  in  one  direction,  being  repre- 
sented by  values  above  the  line  A  E,  and  the  nega- 
(185) 


186  ELECTRICAL  MEASUREMENTS. 

tive  electromotive  forces,  or  those  which  tend  to 
produce  currents  in  the  opposite  direction,  being 
represented  by  values  below  the  line  A  E.  The 
curves  ABC  and  CD  E,  thus  produced,  are  called 
the  phases  of  the  current  A  B  C,  the  one  above  the 
line  being  generally  'called  the  positive  phase,  and  C 
D  E,  the  one  below  the  line,  the  negative  phase. 

The  phase,  which    is  the  time  required  to  com- 
plete the  to-and-fro  motions  above  and  below  the 
B 


0 

FIG.  88.— CURVE  OF  ELECTROMOTIVE  FORCES  OF  ALTERNATING 
CURRENTS. 

line  A  E,  is  practically  the  periodicity  of  the 
machine,  and  is  dependent  on  the  armature  speed 
and  the  number  of  poles  employed  in  the  machine. 

Two  alternating  currents  are  said  to  possess  the 
same  phase  when  the  electromotive  forces  that  pro- 
duce them  are  simultaneously  directed  in  the  same 
direction. 

Two  alternating  currents  are  said  to  possess  the 
same  period  when  the  times  during  which  the  elec- 


ALTERNATING  CURRENTS.  187 

tromotive  forces  tend  to  produce  currents  in  the  same 
direction  are  equal."  Two  alternating  currents  are 
said  to  be  in  synchronism  with  each  other  when 
their  electromotive  forces  tend  to  produce  currents 
in  the  same  direction  and  for  the  same  length  of 
time. 

If  two  alternating  dynamos  be  connected  to  the 
same  leads  in  series,  the  electromotive  force  pro- 
duced in  such  leads  will  theoretically  be  equal  to  the 
sum  of  the  electromotive  forces  of  the  two  machines. 
This,  however,  is  only  true  if  the  phases  of  the  two 
machines  remain  exactly  the  same.  If  they  differ 
in  even  the  slightest  degree,  they  will  rapidly  tend 
to  increase  this  difference  of  phase  until  they  are  in 
exactly  opposite  phases,  when,  of  course,  they  will 
produce  no  current.  Series  connection  or  running 
of  alternators  is,  therefore,  impracticable. 

If,  however,  two  alternators  be  connected  to  the 
same  leads  in  parallel,  then,  provided  the  armature 
circuits  are  so  arranged  that  the  currents  in  them  can 
be  rapidly  reversed,  and  very  small  electromotive 
forces  impressed  on  such  circuits  can  produce  large 
currents  in  them,  and  the  engines  driving  the  dyna- 
mos are  under  the  control  of  the  dynamos,  that  is,  are 
not  governed,  then  such  machines,  even  if  out  of 
synchronism  when  coupled  to  the  leads,  will,  almost 


188  ELECTRICAL  MEASUREMENTS. 

immediately,  pull  each  other  into  parallelism,  each 
promptly  exercising  an  automatic  synchronizing  con- 
trol over  the  other. 

The  marked  advantages  possessed  by  alternating 
currents  for  certain  kinds  of  electrical  work  have 
led  to  an  extended  study  of  their  peculiarities.  Such 
study  has  disclosed  the  fact  that  alternating  currents 
differ  in  marked  respects  from  the  direct  or  continu- 
ous electric  currents  that  have  heretofore  been  almost 
exclusively  employed  in  practical  electrical  work. 

The  following  peculiarities  concerning  alternating 
currents  should  be  carefully  remembered  : 

(1.)  The  direction  of  the  current  undergoes  regu- 
lar changes. 

(2.)  The  strength  of  the  current  undergoes  regu- 
lar reversals. 

(3.)  The  peculiarities  of  the  changes  either  in  the 
direction  of  the  current  or  in  its  strength  during  one 
complete  alternation,  or  one  complete  to-and-fro  mo- 
tion, are  regularly  repeated  during  any  subsequent 
to-and-fro  motion. 

A  motion  that  regularly  recurs  or  reproduces  itself 
at  regular  intervals  according  to  a  certain  law  is 
called,  in  scientific  language,  a  simple  harmonic 
motion,  or  a  simple  periodic  motion. 

A  pendulum  that  is  set  swinging  in  a  circular  path 


ALTERNATING  CURRENTS. 


189 


affords  an  example  of  a  simple-harmonic  motion.  If 
such  a  pendulum  be  looked  at  either  from  above  or 
below,  its  path  will  appear  to  be  circular.  If  looked 
at,  however,  from  one  side,  its  path  will  appear  to  be 
elliptical,  and  such  elliptical  path  will  appear  longer 
and  narrower  as  the  eye  of  the  observer  approaches 
the  level  of  the  plane  in  which  the  bob  of  the  pendu- 
lum moves,  and,  when  it  reaches  this  point,  the  bob 
will  appear  to  move  backward  and  forward  in  a 
straight  line. 


FIG.  89.— SIMPLE-HARMONIC  MOTION. 

Let  the  circle  Q  o  R,  Fig.  89,  represent  the  path 
in  which  the  bob  moves,  and  let  Q  A,  A  B,  B  C,  Co, 
etc.,  be  equal  distances  in  such  path.  Let  the  lines 
A  a,  B  b,  C  c,  o  0,  etc.,  be  drawn  perpendicular  to 
the  line  Q  R.  Then,  when  looked  at  with  the  eye 
in  the  plane  in  which  the  bob  travels,  the  line  Q  R, 
will  be  the  path  in  which  the  bob  appears  to  move 


190  ELECTRICAL  MEASUREMENTS. 

backward  and  forward,  and  the  lines  Qa,  a  b,  b  c, 
c  0,  etc.,  will  represent  the  spaces  apparently  trav- 
ersed in  equal  intervals  of  time. 

The  circle  Q  o  R,  is  called  the  circle  of  reference. 

By  a  simple-harmonic  motion  is  meant  the  motion 
which  would  result  from  the  projection  of  the  circu- 
lar motion  of  the  pendulum  on  the  diameter  Q  0  R, 
or  the  motion  that  is  executed  backward  and  for- 
ward along  the  line  Q  0  R. 

Examples  of  approximately  simple-periodic  or 
simple-harmonic  motion  are  seen  in  the  movements 
of  the  connecting  rod  of  an  ordinary  steam  engine 
or  in  the  motion  of  the  piston-rod  of  the  steam 
engine. 

The  regularity  with  which  both  the  variations  in 
the  strength  of  the  alternating  current  and  the 
changes  in  its  direction  recur,  render  the  motions  of 
such  currents  simple-harmonic  or  simple-periodic 
motions.  Alternating  currents  are  sometimes  called 
simple-harmonic  or  simple-periodic  currents.  In 
many  cases,  however,  the  motions  of  alternating  cur- 
rents partake  of  the  nature  of  complex-harmonic 
motions. 

In  the  case  of  a  direct  or  continuous  current,  the 
current  strength  is  equal  to  the  electromotive  force 
divided  by  the  resistance.  In  the  case  of  an  alter- 


ALTERNATING  CURRENTS.  191 

nating  current,  since  the  electromotive  force  is  con- 
stantly undergoing  a  change  both  in  value  and  direc- 
tion, to  determine  the  current  strength  produced 
the  average  electromotive  force  must  be  divided  by  a 
quantity,  allied  to  resistance,  called  impedance. 

It  is  well  known  in  the  phenomena  of  electro- 
dynamic  induction,  that  when  the  current  strength 
in  any  circuit  undergoes  variations  the  expanding 
and  contracting  lines  of  magnetic  force  cut  portions 
of  the  circuit  and  produce  electromotive  forces,  that 
tend  to  produce  a  current  in  one  direction  when 
such  lines  of  force  are  moving  inward  or  toward  the 
conductor,  and  in  the  opposite  direction  when  they 
are  moving  outward  or  from  the  conductor.  Now 
this  action  of  the  current  in  inducing  a  current  on 
itself,  while  its  strength  or  duration  is  changing, 
which  is  called  self-induction,  or  more  simply  in- 
ductance, exists  in  a  marked  degree  in  the  alter- 
nating current.  In  any  circuit,  whatever  be  the  kind 
of  the  current  flowing  through  it,  the  passage  of  the 
current  is  resisted  or  opposed  by  the  resistance  of 
the  conductor.  In  the  case  of  an  alternating  current, 
besides  this  resistance  there  exists  another  apparent 
resistance  which  opposes  the  passage  of  the  current ; 
namely,  the  self-induction  or  inductance  produced 
by  an  electromotive  force  acting  in  such  a  direction 


192  ELECTRICAL  MEASUREMENTS. 

as  to  oppose  the  passage  of  the  current.     This  is 
sometimes  called  the  spurious  resistance. 

Impedance  in  any  circuit  may  be  defined  generally 
as  opposition  to  current  flow.  The  impedance  is 
equal  to  the  sum  of  the  inductance  and  the  ohmic 
resistance  arising  from  the  dimensions  and  character 
of  the  conductor.  In  the  case  of  any  direct  or  con- 
tinuous current,  C,  the  current  strength  equals  E, 
the  electromotive  force,  divided  by  R,  the  resistance  ; 
or, 


OHMIC        RESISTANCE 
FIG.  90.-GEOMETRICAL  REPRESENTATION  OF  IMPEDANCE. 

In  a  simple-periodic  or  alternating  current  the 
average  current  strength 

_  the  average  impressed  electromotive  force 
impedance. 

The  impedance  is  a  quantity  equal  to  the  square 
root  of  the  sum  of  the  squares  of  the  inductive  resist- 
ance of  the  circuit  and  the  ohmic  resistance. 

The  impedance  of  a  circuit  can  be  represented 


ALTERNATING  CURRENTS.  193 

geometrically,  as  shown  in  Fig.  90.  If  the  base  of 
the  right-angled  triangle  represents  the  ohmic  resist- 
ance, and  the  perpendicular  height  the  inductive 
resistance,  then  the  hypotenuse,  which  is  equal  to 
the  square  root  of  the  sum  of  the  squares  of  the  base 
and  the  perpendicular,  equals  the  impedance. 

A  rapidly  alternating  current  produces  a  variety 
of  phenomena  during  its  passage  through  a  conduc- 
tor, which  differ  markedly  from  the  passage  of  a 
direct  or  continuous  current  through  the  same  con- 
ductor. When  a  steady  current  flows  through  a  con- 
ductor, the  current  density-is  the  same  for  all  areas 
of  cross  section.  With  a  rapidly  alternating  current, 
however,  the  current  density  is  greater  near  the  sur- 
face, and,  when  the  rate  of  alternation  is  sufficiently 
great,  the  current  is  almost  entirely  absent  from  the 
central  portions  of  the  conductor. 

It  has  been  shown  by  Lord  Eayleigh  that  when  the 
rate  of  alternations  is  1,050  per  second,  the  resist- 
ance of  a  conductor  100  millimeters  in  length  and 
30  millimeters  in  diameter  is  1.84  times  its  resistance 
to  direct  or  continuous  currents.  He  found  that 
such  increase  of  resistance  was  greater  with  conduc- 
tors of  great  diameter  than  with  those  of  small 
diameter. 

A  careful  study  of  some  of  the  peculiarities  of 


194  ELECTRICAL  MEASUREMENTS. 

alternating  currents  has  led  to  a  radical  change  of 
opinion  concerning  the  manner  in  which  the  current 
is  believed  to  flow  through  conducting  paths.  The 
current  is  not  supposed  to  flow  through  the  conduc- 
tor, but  to  be  propagated  through  the  ether  or  other 
di-electric  which  surrounds  the  conductor  and  -lies 
outside  it.  The  conductor  merely  acts  as  a  sink  or 
place  where  the  energy  of  the  current  is  rained  down 
upon  it. 

The  current,  or,  perhaps,  more  correctly  speaking, 
that  which  results  in  the  current,  is  regarded  as 
beginning  at  the  surface  of  the  conductor  and 
more  or  less  slowly  soaking  tli rough  it  toward  the 
centre.  If  the  current  is  direct  or  continuous  it 
soon  reaches  the  deepest  layers  of  the  conduc- 
tor; but  if  it  is  rapidly  alternating,  before  it 
can  soak  very  far  into  the  conductor  toward  its  cen- 
tre it  is  turned  back  toward  its  surface,  and  so  be- 
comes confined  to  layers  which  will  become  more 
and  more  superficial  as  the  rapidity  of  the  reversals 
increases. 

The  conception  of  a  rapidly  alternating  current 
flowing  through  a  conductor  by  starting  at  the  sur- 
face and  gradually  soaking  in  toward  the  centre 
does  not  regard  the  electric  energy  as  moving 
through  the  conductor  after  the  manner  of  water 


ALTERNATING  CURRENTS.  195 

flowing  through  pipes,  but  as  actually  being  rained 
down  on  its  surface  from  the  space  outside  of  it. 

This  conception  concerning  the  flow  of  alternating 
currents  through  a  conductor  is  not  unlike  our  idea 
of  the  flow  of  heat  through  a  conducting  wire,  as  has 
been  pointed  out  by  Stephan.  If,  for  example,  a 
wire  or  conductor  which  has  been  uniformly  heated 
throughout  be  suddenly  carried  into  a  space  where 
the  temperature  is  higher  than  itself,  the  heat  energy 
will  pass  into  such  wire  from  the  surface  toward  the 
interior,  the  outer  portions  of  the  wire  first  rising  in 
temperature  and  afterward  the  inner  portions.  In 
other  words,  in  the  case  of  a  wire  whose  area 
of  cross  section  is  circular  the  heat  penetrates 
toward  the  centre  in  successive  concentric  layers; 
or,  conversely,  if  such  a  wire  be  carried  into  a  space 
whose  temperature  is  lower  than  the  temperature 
of  the  wire,  the  wire  or  conductor  parts  with  its  heat 
energy  by  a  movement  which  takes  place  through 
the  wire  outward. 

Now,  when  the  ends  of  a  cylindrical  conductor  are 
subjected  to  an  alternating  electromotive  force,  by 
connection  with  the  terminals  of  an  alternating  cur- 
rent dynamo,  the  energy  of  the  current  is  believed 
to  be  conveyed  to  such  conductor,  not  by  actual 
passage  through  its  substance,  but  through  the  space 


196  ELECTRICAL  MEASUREMENTS. 

outside  the  conductor,  said  energy  being  rained 
down  on  its  surface  from  the  exterior,  and  gradually 
penetrating  said  conductor  toward  its  centre.  It 
will  be  seen  that,  in  reality,  a  solid  cylindrical  con- 
ductor, which  is  conveying  rapidly  alternating  cur- 
rents, may  in  reality  be  regarded  as  a  hollow  cylin- 
der of  the  same  dimensions  as  the  solid  conductor, 
the  thickness  of  the  material  in  which  will  become 
smaller  and  smaller  as  the  rapidity  of  alternation 
increases. 

This  conception  concerning  the  passage  of  cur- 
rents through  conductors  was  first  suggested  by 
Poynting,  and  is  known  as  Poyn ting's  law. 

The  discharges  of  a  Leyden  jar  partake  of  the  nat- 
ure of  very  rapidly  alternating  discharges.  When 
such  discharges  are  passed  through  the  primaries  of 
specially  devised  induction  coils,  as  had  been  done  by 
Nikola  Tesla  and  Elihu  Thomson,  they  produce  all 
the  phenomena  of  rapidly  alternating  currents  in 
secondaries  placed  near  them.  As  the  electromotive 
force  of  such  discharges  is  extremely  high,  it  has 
been  found  necessary  in  practice  to  insulate  the  pri- 
mary and  secondary  coils  from  each  other  by  oil. 

The  phenomena  of  the  alternative  path  of  a  dis- 
charge taken  from  a  Leyden  jar  also  demonstrates  its 
oscillatory  or  alternating  character.  Such  phenom- 


ALTERNATING  CURRENTS.  197 

ena  are  seen  in  the  lateral  discharges  that  occur 
through  the  air  space  that  separates  such  conductors 
from  neighboring  conductors.  If,  for  example,  a 
Leyden  jar  is  provided  with  discharge  wires  or  con- 
ductors, as  shown  in  Fig.  91,  the  discharge  is  si- 
multaneously attended  by  a  large  spark  at  B,  between 
the  ends  of  two  long  open-circuited  leads. 

The  oscillations  produced  by  the  discharge  in  A, 
produce  a  pulsating  field  which  induces  oscillatory 
discharges  in  the  open-circuited  leads  B.  The  coun- 
ter-electromotive force  produced  in  a  conductor, 
through  which  an  oscillating  discharge  is  passing,  by 


-JJ 

FIG.  91.— PHENOMENA  OF  ALTERNATIVE  PATH. 

the  induction  of  the  circuit  on  itself  may  render  such 
conductor  of  much  higher  resistance  than  an  inter- 
vening air  space  through  which  the  discharge  now 
takes  place.  These  principles  have  been  applied  by 
Lodge  and  others  to  the  construction  and  placing  of 
lightning  conductors  so  as  to  best  enable  them  to  act 
more  efficiently  in  carrying  the  discharge  from  the 
neighboring  cloud  to  the  earth. 

If  the  oscillatory  discharge  of  a  Leyden  jar  be 


198  ELECTRICAL  MEASUREMENTS. 

sent  through  a  magnetizing  coil,  the  magnetization 
so  produced  on  a  steel  rod  placed  inside  such  coil  is 
of  so  curious  a  nature  that  it  was  formerly  described 
as  anomalous  magnetization.  In  such  cases  the  bar 
is  not  magnetized  in  the  manner  it  would  be  if  the 
current  passing  through  the  magnetizing  coil  were 
direct  or  continuous,  but  exhibits  zones  of  positive 
and  negative  magnetization  extending  from  the  sur- 
face of  the  bar  inward  toward  the  centre.  The  ex- 
planation of  this  apparent  anomaly  is  readily  under- 
stood when  the  alternating  character  of  a  Leyden 
jar  discharge  is  borne  in  mind.  In  other  words,  it 
is  not  the  magnetization  that  is  anomalous.  If 
there  be  any  anomaly  it  is  rather  to  be  found  in  the 
alternating  or  oscillating  of  the  d  scharge  of  the 
Leyden  jar. 

In  order  to  avoid  the  effects  of  induction  produced 
in  conductors  'which  lie  near  other  conductors, 
through  which  alternating  currents  of  electricity  of 
very  high  frequencies  are  passing,  it  is  only  necessary 
to  place  on  the  outside  of  such  conductors  a  con- 
ducting coating  or  covering  of  metal  insulated  there- 
from. The  thickness  of  said  conductor  will  neces- 
sarily depend  on  its  conducting  power.  This  thick- 
ness, however,  will  be  smaller  in  proportion  as  the 
rapidity  of  the  alternations  increases.  As  a  rule,  lead. 


ALTERNATING  CURRENTS.  199 

coated  conductors  do  not  act  efficiently  for  such  pur- 
poses on  account  of  the  low  conducting  power  of  lead. 

The  manner  in  which  a  conducting  plate  of  metal 
acts  to  protect  the  conductor  from  the  effects  of  a 
neighboring  conductor  through  which  a  rapidly 
alternating  current  is  passing,  when  such  a  plate  is 
placed  between  two  conductors,  is  as  follows  :  Before 
the  expanding  or  contracting  lines  of  force  can  cut 
or  pass  through  the  neighboring  conductor,  they 
expend  their  energy  in  producing  currents  by  in- 
duction in  the  material  of  the  interposed  plate.  If 
such  plate  is  of  sufficient  thickness,  all  their  energy 
is  expended  on  the  interposed  plate,  and  the  neigh- 
boring conductor  is  screened  or  protected  from  such 
action. 

In  other  words,  the  screening  is  due  to  the  pro- 
duction in  the  interposed  plate  of  currents  called 
eddy  currents,  as  can  be  proved  by  the  fact  that 
when  such  screen  is  split  radially  from  the  circnm- 
ference  to  the  centre,  its  screening  effect  is  removed, 
since  then  such  eddy  currents  cannot  be  produced. 

When  the  screening  plate  is  formed  of  iron  it 
produces  an  additional  effect  from  the  tendency  of 
such  a  plate  to  condense  the  lines  of  magnetic  force 
on  it  by  reason  of  the  small  magnetic  resistance 
which  iron  offers  to  their  passage  through  it. 


200  ELECTRICAL  MEASUREMENTS. 

The  magnetic  screens  used  for  watches  consist  es- 
sentially of  iron  cases,  which  protect  the  magnet- 
ized portions  of  the  works  by  closing  the  lines  of 
magnetic  force  through  the  iron  case. 


ALTERNATING  CURRENTS.  201 


EXTRACTS  FROM  STANDARD   WORKS. 

The  conditions  which  are  now  generally  assumed 
to  attend  the  transmission  of  an  alternating  current 
through  a  conductor  are  thus  happily  expressed  by 
Fleming  in  his  "  The  Alternate  Current  Trans- 
former/'* Vol.  I.,  on  page  476: 

The  whole  history  of  the  discharge  may  be  divided  into 
three  parts.  First,  a  time  when  the  energy  associated  with 
the  system  is  nearly  all  electrostatic  and  is  represented  by 
the  energy  of  the  lines  or  tubes  of  electrostatic  induction 
running  from  plate  to  plate  ;  second,  a  period  when  the  dis- 
charge is  at  its  maximum,  when  the  energy  exists  partly 
as  energy  associated  with  lines  of  electrostatic  induction 
expanding  outward,  and  partly  in  the  form  of  closed  rings 
or  tubes  of  magnetic  force  expanding  and  contracting  back 
on  the  wire  ;  and  then,  lastly,  a  period  when  nearly  all  the 
energy  has  been  absorbed  or  buried  in  the  wire,  and  has 
there  been  dissipated  in  the  form  of  heat,  which  is  radiated 
out  again  as  energy  of  dark  or  luminous  radiation.  The 
function  of  the  discharging  wire  is  to  localize  the  place  of 
dissipation  and  also  to  localize  the  place  where  the  mag- 
netic field  shall  be  most  intense,  and  all  that  observation  is 
able  to  tell  us  about  a  conductor  which  is  conveying  that 

*  "The  Alternate  Current  Transformer  in  Theory  and  Practice," 
by  J.  A.  Fleming,  M.  A.,  D.  Sc.  2  vols.  London:  The  Electrician 
Printing  and  Publishing  Co.  1889-1892.  V.M.  I.,  487  pages,  157  illus- 
trations. Price  $3.00.  Vol.  II.,  594  pages,  300  illustrations.  Price 
$5.00. 


202  ELECTRICAL  MEASUREMENTS. 

which  is  called  an  electric  current  is  that  it  is  a  place  where 
heat  is  being  generated  and  near  which  there  is  a  magnetic 
field.  These  conceptions  lead  us  to  fresh  views  of  very 
familiar  phenomena.  Suppose  we  are  sending  a  current 
of  electricity  through  a  submarine  cable  by  a  battery, 
say,  from  zinc  to  earth,  and  suppose  the  sheath  is 
everywhere  at  zero  potential,  then  the  wire  will  be  every- 
where at  a  higher  potential  than  the  sheath,  and  the  level 
surface  will  pass  through  the  insulating  material  to  the 
points  where  they  cut  the  wire.  The  energy  which  main- 
tains the  current,  and  which  works  the  needle  at  the  fur- 
ther end  travels  through  the  insulating  material,  the .  core 
serving  as  a  means  to  allow  the  energy  to  get  into  motion 
or  to  be  continually  propagated.  This  energy  sucked  up 
by  the  core  is,  however,  transformed  into  heat  and  radi- 
ated again  as  dark  heat.  If  we  adopt  the  electro-magnetic 
theory  of  light,  it  moves  out  again  still  as  electro-magnetic 
energy,  but  in  a  different  form,  with  a  definite  velocity 
and  intermittent  in  type.  We  have,  then,  in  the  case  of  the 
electric  light  this  curious  result — that  energy  moves  in  upon 
the  arc  or  filament  from  the  surrounding  medium,  there  to 
be  converted  into  a  form  in  which  it  is  sent  out  again,  and 
through  which  the  same  in  kind  is  able  to  affect  our 


In  the  case  of  an  arc  or  glow-lamp  worked  by  an  alter- 
nating current,  we  have  still  further  the  result  that  the 
energy  which  moves  in  the  carbon  is  returned  again,  with 
no  other  change  than  that  of  a  shortened  wave-length,  and 
the  carbon  filament  performs  the  same  kind  of  change  on 
the  electro-magnetic  radiation  as  is  performed  when  we 


ALTERNATING  CURRENTS.  203 

heat  a  bit  of  platinum  foil  to  vivid  incandescence  in  a  focus 
of  dark  heat.  A  current  through  a  seat  of  electromotive 
force  is  therefore  a  place  of  divergence  of  energy  from  the 
conducting  circuit  into  the  medium,  and  this  energy  travels 
away  and  is  converged  and  transformed  by  the  rest  of  the 
circuit.  From  this  aspect  the  function  of  the  copper  con- 
ducting wire  fades  into  insignificance  in  interest  in  com- 
parison with  the  function  of  the  dielectric,  or  rather  of  the 
ether  contained  in  the  dielectric.  When  we  see  an  electric 
tramcar,  or  motor,  or  lamp  worked  from  a  distant  dynamo, 
these  notions  invite  us  to  consider  the  whole  of  that  energy, 
even  if  it  be  thousands  of  horse-power  per  hour,  as  con- 
veyed through  the  ether  or  magnetic  medium,  and  the  con- 
ductor as  a  kind  of  exhaust  valve,  which  permits  energy  to 
be  continually  supplied  to  the  dielectric. 

In  his  "  Dictionary  of  Electrical  Words,  Terms 
and  Phrases/'  *  on  page  467,  the  author  thus  defines 
and  explains  magnetic  screening: 

Screening,  Magnetic. — Preventing  magnetic  induction 
from  taking  place  by  interposing  a  metallic  plate,  or  a 
closed  circuit  of  insulated  wire,  between  the  body  produc- 
ing the  magnetic  field  and  the  body  to  be  magnetically 
screened. 

A  magnetic  needle  is  screened  from  the  action  of  the 
earth's  field  by  placing  it  inside  a  hollow  iron  box,  which 

*"A  Dictionary  of  Electrical  Words,  Terms  and  Phrases."  by 
Edwin  J.  Houston,  A.M.  Second  edition.  New  York :  The  W.  J. 
Johnston  Company,  Ltd.  1892.  562  pages,  510  illustrations.  Price 
15.00. 


204  ELECTRICAL  MEASUREMENTS. 

prevents  the  lines  of  force  of  the  earth's  field  from  passing 
through  it  by  concentrating  them  on  itself.  This  action 
is  dependent  on  the  fact  that  iron  is  paramagnetic  and 
therefore  offers  the  lines  of  force  less  resistance  through 
its  mass  than  elsewhere.  A  plate  of  copper  would  not 
effect  any  such  magnetic  shielding  or  screening.. 

In  any  magnetic  field,  however,  in  which  the  strength  of 
the  field  is  undergoing  rapid  periodic  variations,  a  plate  of 
copper  or  other  electric  conductor  may  act  as  a  screen  to 
protect  neighboring  conductors  from  the  effects  of  magnetic 


FIG.  92. 

induction,  audits  ability  to  thoroughly  effect  such  a  screen- 
ing will  depend  directly  on  its  conducting  power. 

If,  for  example,  the  copper  plate  c,  (Fig.  92)  be  inter- 
posed between  a  coil  of  copper  ribbon  a,  and  the  fine  wire 
coil  b,  it  will  greatly  reduce  the  intensity  of  the  induced 
currents  produced  when  rapidly  alternating,  currents  are 
sent  through  a.  If,  however,  the  copper  plate  be  slit,  as 
shown  to  the  right  at  a,  the  screening  effect  is  lost,  but 
is  regained  if  the  slit  be  connected  by  a  conductor. 
Similarly  a  flat  coil  of  insulated  wire  effects  no  screening 


ALTERNATING  CURRENTS.  205 

action  when  open,  but  when  closed  acts  as  the  uncut  copper 
plate. 

Here  the  screening  action  is  due  to  the  fact  that  the  en- 
ergy of  the  field  is  spent  in  producing  eddy  currents  in  the 
interposed  metal  screen  or  coils.  If  the  metal  screen  is 
discontinuous  in  the  direction  in  which  the  eddy  currents 
tend  to  flow,  the  inability  of  the  screen  to  absorb  the  en- 
ergy as  eddy  currents  prevents  its  action  as  a  screen. 

The  term  magnetic  screening  is  generally  employed  in 
the  latter  sense  of  preventing  magnetic  induction  from  oc- 
curring in  a  neighboring  conductor,  by  interposing  some 
conducting  substance  in  which  eddy  currents  can  be  freely 
established. 

As  to  the  efficiency  of  the  screening  action,  if  the  makes- 
and-breaks  do  not  follow  one  another  very  rapidly,  the  fol- 
lowing principles  can  be  proved  : 

(1.)  If  the  screening  material  have  absolutely  no  electrical 
resistance  it  will  effect  a  perfect  magnetic  screening  when 
placed  between  the  primary  and  secondary,  no  matter  what 
its  thickness  may  be. 

(2.)  If  the  screed  have  a  finite  conductivity  the  screening 
will  be  imperfect,  unless  the  thickness  of  the  material  em- 
ployed is  considerable. 

If,  however,  the  makes-and-breaks  follow  one  another 
very  rapidly,  then 

The  screening  effect  of  even  imperfect  conductors  will 
become  manifest  with  comparatively  thin  screens  of  metal. 

As  to  magnetic  screening,  therefore,  it  follows  that  the 
less  the  conductivity  the  greater  must  be  the  speed  of  re- 
versal, in  order  that  the  screening  action  may  be  effective. 


206  ELECTRICAL  MEASUREMENTS. 

Where  a  screen  of  iron  is  employed,  an  additional  effect 
is  produced  by  the  fact  that  the  small  magnetic  resistance 
of  the  metal,  or  its  conductivity  for  lines  of  magnetic  force, 
causes  the  lines  of  induction  to  pass  through  its  mass,  and 
thus  effect  a  screening  action  for  the  space  on  the  other 
side.  This  action  is,  by  some,  called  magnetic  screening. 

In  the  case  of  iron  screens,  considerable  thickness  is  re- 
quired in  the  metal  plate,  hi  order  to  obtain  efficient  screen- 
ing action  of  this  latter  character.  On  account  of  this  ac- 
tion of  iron,  in  conducting  away  lines  of  force,  a  much 
smaller  speed  of  reversal  is  required  in  order  to  obtain  ef- 
fective screening  action  where  plates  of  iron  are  used  than 
in  the  case  of  plates  of  other  metal. 


FIG.  93.— WlLLOUGHBY  SMITH'S  APPARATUS. 

The  apparatus  shown  in  Fig.  93  was  employed  by  Mr. 
Willoughby  Smith  in  studying  the  effects  of  magnetic 
screening. 

The  flat  coils  A  and  B,  were  employed  for  the  primary 
and  secondary  coils  respectively,  and  were  connected  to  the 
battery  C,  and  the  galvanometer  F,  as  shown.  Current  re- 
versers  D  and  E,  were  so  arranged  as  to  reverse  galva- 
nometer and  battery  alternately,  and  so  cause  the  opposite 
induced  currents  to  affect  the  galvanometer  in  the  same 
direction. 


X.—ALTERNA  TING  CURRENT  DISTRIBU- 
TION. 


Some  of  the  peculiarities  of  alternating  currents 
render  their  employment  for  the  distribution  of 
electric  energy  over  extended  areas  very  advanta- 
geous in  the  case  of  many  of  the  practical  applica- 
tions of  electricity. 

A  system  for  the  distribution  of  electricity  by 
means  of  alternating  currents  embraces  the  follow- 
ing parts,  namely  : 

(1.)  An  alternating  current   dynamo-electric  ma- 
chine, or  battery  of  machines. 
"  (2).  A  pair  of  conductors  or  line  wires  arranged  in 
a  metallic  circuit. 

(3.)  A  number  of  transformers  whose  primary 
coils  are  placed  in  the  circuit  of  the  lirie  wires. 

(4.)  A  number  of  electro-receptive  devices  placed 
in  the  circuit  of  the  secondary  coils  of  the  trans- 
formers. 

(5.)  Instruments  for  maintaining  constant  either 
the  current  or  the  potential,  despite  changes  in  the 
load  on  the  consumption  circuit. 
(207) 


208  ELECTRICAL  MEASUREMENTS. 

There  are  two  distinct  systems  of  distribution  by 
means  of  which  the  effects  of  alternating  currents 
can  be  employed  at  points  situated  at  fairly  con- 
siderable distances  from  where  the  sources  are  lo- 
cated, namely  : 

(1.)  A  system  of  constant  potential  distribu- 
tion. 

Here  the  primary  coils  of  a  number  of  transformers 
are  connected  in  multiple  to  leads  which  connect  the 
distant  stations  where  the  source  of  alternating  cur- 
rents is  located. 

Such  leads  are  maintained  at  an  approximately 
constant  average  electromotive  force,  or  potential. 
In  constant  potential  distribution  the  primary  cir- 
%cuits  of  the  transformers  consist  of  a  great  length  of 
comparatively  thin  wire  and  the  secondary  coils  of  a* 
comparatively  short  length  of  thick  wire.  This  sys- 
tem of  distribution  is  especially  adapted  for  the  opera- 
tion of  incandescent  lamps  or  other  receptive  devices 
that  are  connected  to  the  leads  or  conductors  in  mul- 
tiple. 

(2.)  A  system  of  constant  current  distribution. 

Here  the  primary  coils  of  a  number  of  transformers 
are  connected  in  series  in  a  single  metallic  circuit. 
In  this  case  the  primary  coils  consist  of  short  thick 
wires  and  the  secondary  of  long  thin  wires.  Such  a 


ALTERNATING  CURRENT  DISTRIBUTION.       209 

system  is  especially  adapted  for  the  distribution  of 
arc  lights. 

Let  us  now  examine  the  advantages  to  be  derived 
from  the  distribution  of  electricity  by  means  of 
alternating  currents  employed  in  connection  with 
leads  maintained  at  approximately  constant  differ- 
ences of  potential,  in  cases  where  the  distance  be- 
tween that  part  of  the  line  where  such  currents  are 
generated  and  that  part  at  which  they  are  to  be  util- 
ized is  fairly  considerable. 

During  the  passage  of  an  electric  current  through 
any  circuit,  the  energy  which  is  expended  in  any  par- 
ticular parts  of  the  circuit  is  in  direct  proportion  to 
the  resistance  of  such  parts.-  Now,  in  any  system  of 
distribution  economy  of  distribution  requires  that 
the  energy  expended  in  the  electro- receptive  devices 
shall  bear  as  large  a  proportion  as  possible  to  the  en- 
ergy expended  in  the  entire  circuit.  No  difficulty 
exists  in  obtaining  economical  conditions  in  series- 
distribution  circuits,  even  though  their  length  is 
considerable,  for  the  total  resistance  of  such  a  cir- 
cuit increases  with  every  electro-receptive  device 
added.  Therefore,  even  though  comparatively  thin 
wires  of  great  resistance  are  employed  to  connect 
such  separate  electro-receptive  devices  in  series,  yet 
the  greater  proportion  of  such  resistance  can  still 


210  ELECTRICAL  MEASUREMENTS. 

be  placed  in  the  devices  themselves,  and,  therefore, 
a  considerable  extent  of  territory  can  be  covered  by 
such  circuits  without  very  great  loss. 

In  systems  of  multiple  distribution,  however, 
an  indefinite  increase  in  the  number  of  electro-re- 
ceptive devices  is  impossible,  because  every  electro- 
receptive  device  added  decreases  the  total  resistance 
of  the  circuit,  and,  unless  the  resistance  of  the  leads 
is  correspondingly  decreased,  the  economy  becomes 
smaller,  unless,  of  course,  the  resistance  of  the  leads 
was  originally  so  low  as  to  be  inappreciable  when 
compared  with  the  change  of  resistance. 

In  systems  of  distribution  by  means  of  transform- 
ers, alternating  currents,  of  small  current  strength 
and  considerable  difference  of  potential,  are  sent 
over  a  line  from  a  distant  station,  and  passing 
through  the  primary  coils  of  a  number  of  step-down 
transformers,  generally  connected  to  the  line  in  mul- 
tiple-arc, produce,  by  induction,  currents  of  compar- 
atively great  strength  and  small  difference  of  poten- 
tial in  the  secondary  coils.  The  electro-receptive 
devices  are  connected  in  multiple-arc  to  the  circuits 
connected  to  the  terminals  of  the  secondary  coils. 

In  cases  where  the  distance  is  considerable,  this 
method  of  distribution  is  employed  and  greatly  re- 
duces the  cost  of  the  main  conducting  wires  or 


ALTERNATING  CURRENT  DISTRIBUTION.       211 


leads,  because  the  current  strength  in  the  line  wire 
is  very  small  as  compared  with  its  value  after  con- 
version. 

The  general  arrangement  of  the  transformers  on 
the  main  line,  the  secondary  circuits  and  the  con- 
nection of  the  electro-receptive  devices  "to  the  coils 
of  the  secondary  circuits  are  shown  in  Fig.  94. 
The  transformers  are  supported  on  line  poles,  as 


FIG.  91  .—DETAILS  OF  TRANSFORMER  CIRCUITS. 

more  clearly  shown  in  Fig.  95,  in  which  the  ter- 
minals of  the  primary  and  secondary  of  the  trans- 
former are  readily  seen. 

When  the  primary  circuits  of  the  transformers  are 
connected  in  multiple-arc  to  leads  or  conductors  that 
are  maintained  at  approximately  constant  differences 
of  potential,  and  the  lamps  or  receptive  devices  are 
also  connected  to  the  leads  in  multiple,  then,  when 


212 


ELECTRICAL  MEASUREMENTS. 


the  impedance  of  the  primaries  of  each  transformer 
is  large  enough,  when  its  secondary  is  open,  to  keep 
practically  all  current  from  the  primary,  the  amount 
of  current  which  can  flow  through  the  primary  cir- 
cuit of  such  transformers  is  automatically  limited  to 


FIG.  95.— PRIMARY  AND  SECONDARY  CIRCUITS  OP  TRANSFORMERS. 

the  actual  requirements  of  the  devices  connected  in 
multiple-arc  to  the  secondary  circuits  of  the  trans- 
formers, and  the  system  becomes  automatically  self- 
regulating. 


ALTERNATING  CURRENT  DISTRIBUTION.       213 

If,  for  example,  a  full  load  is  placed  on  such  a 
line,  and  lamp  after  lamp  is  turned  out,  the  current 
in  the  primary  will  decrease,  a  smaller  current  pass- 
ing into  the  primary  from  the  constant  potential 
leads,  and,  at  last,  when  all  such  lamps  are  turned 
off,  although  the  primary  circuit  is  still  connected 
as  before  to  the  mains  at  full  pressure,  yet  no  cur- 
rent will  flow  through  such  primary,  and,  therefore, 
no  current  will  be  induced  in  the  secondary. 

The  explanation  of  this  curious  automatic  regu- 
lation is  to  be  found  in  self  induced  counter-electro- 
motive forces  set  up  in  the  primary,  which  effec- 
tually bar  the  current  from  flowing  through  the 
mains. 

When  the  lamps  are  entirely  removed  from 
the  secondary  circuit,  the  secondary  circuit  being 
open,  the  primary  circuit  can  generate  no  current 
therein.  The  lines  of  force  of  the  alternating  pri- 
mary circuit  then  expend  their  energy  on  the  iron 
core  placed  inside  the  primary,  which  sets  up 
counter-electromotiVe  forces  therein  which  act  as 
a  resistance  to  prevent  any  but  a  very  small  current 
from  flowing  through  the  primary  from  the  leads. 

There  may  be  some  difficulty  in  understanding 
why  alternating  currents  should  produce  their  max- 
imum effect,  as  to  the  magnetization  of  the  core,  when 


214  ELECTRICAL  MEASUREMENTS. 

the  lamps  are  not  in  action.  The  reason  is  given 
by  S.  P.  Thompson  substantially  as  follows:  The 
currents  in  the  secondary  are  transformed  into 
almost  exactly  opposite  phases  as  those  in  the 
primary.  While,  therefore,  the  strength  of  the 
current  in  the  primary  is  increasing,  the  strength  of 
the  current  in  the  secondary  circuit  is  increasing  in 
the  opposite  direction  ;  that  is,  the  current  flows 
through  the  secondary  circuit  in  practically  the  oppo- 
site direction  to  what  it  does  through  the  primary  ; 
and,  of  course,  the  magnetizing  effect  on  the  core  is 
less.  •  Therefore,  as  the  load  of  lamps  increases  the 
magnetizing  effect  on  the  core  decreases,  and  the 
counter-electromotive  forces  induced  in  the  primary 
decrease  and  thus  permit  a  stronger  current  to  flow 
therein. 

There  occurs,  however,  a  drop  of  potential  on  the 
mains  conveying  alternating  currents  which  in- 
creases with  the  load.  When  the  variations  in  the 
load  are  great  the  drop  is  sufficiently  great  to  need  an 
increase  of  potential  at  the  generating  end  of  the 
line.  This  increase  is  obtained  either  by  an  increase 
in  the  charge  of  the  field  or  by  the  use  of  a  regu- 
lator. In  the  Stillwell  regulator,  a  transformer,  pro- 
vided with  a  secondary  of  variable  length,  is  so  con- 
nected with  the  line  as  to  add  its  electromotive  force 


ALTERNATING  CURRENT  DISTRIBUTION.       215 

thereto.  The  required  length  of  secondary  is  intro- 
duced into  the  circuit  by  means  of  a  lever  switch. 
When  such  a  regulator  is  used  a  compensator  is  re- 
quired for  the  station  voltmeter. 

The  electricity  required  for  use  in  systems  of  dis- 
tribution by  alternating  currents  is  generally  ob- 
tained by  means  of  an  alternating  current  dynamo- 
electric  machine,  or  by  a  battery  of  such  machines. 
The  rapidity  of  alternation  generally  employed*  for 
such  purposes  is  comparatively  great.  To  readily 
obtain  such  rapidity  of  alternation  it  is  necessary 
that  the  armature  rotate  very  rapidly  in  the  magnetic 
field  of  the  field  magnets  so  as  to  increase  the  num- 
ber of  times  it  alternately  passes  north  and  south 
poles  in  such  field. 

The  rate  of  alternation  required  in  practice  is  so 
great  that  with  a  single  pair  of  poles  in  the  field  of 
the  dynamo  the  peripheral  speed  of  the  armature 
would  be  inconveniently  great.  In  order  to  avoid 
this  some  form  of  mtiltipolar  machine  is  used. 

Various  forms  are  given  to  alternating  current  dy- 
namo-electric machines.  Any  form  of  bi-polar 
or  multi-polar  dynamo-electric  machine  is  capable 
of  producing  alternating  currents  provided  its  field 
magnet  coils  are  suitably  excited.  It  is  only  neces- 
sary to  connect  all  the  positive  ends  of  the  arma- 


216  ELECTRICAL  MEASUREMENTS. 

ture  coils  in  a  certain  position  of  rotation  to  a 
conducting  ring  fixed  to  the  shaft  of  the  machine, 
and  all  the  negative  ends  of  the  coils  to  another 
similar  ring,  and  to  carry  off  the  alternating 
currents  therein  produced  by  means  of  collect- 
ing brushes  resting  on  such  rings.  These  rings 
will  then  become  alternately  positive  and  negative 
on  the  rotation  of  the  armature,  and  will,  therefore, 
furnish  alternating  currents  to  the  brushes  resting 
on  them. 

It  has  been  found  in  practice,  however,  most  con- 
venient to  devise  special  forms  of  dynamo-electric 
machines  for  the  production  of  alternating  currents. 
In  a  well-known  form  of  alternating  current  dynamo- 
electric  machine,  the  field  magnet  frames  contain 
a  series  of  magnet  poles  which  project  radially  in- 
ward from  the  inside  of  the  frame.  These  poles 
are  alternately  of  north  and  south  polarity. 

The  armature  is  formed  of  a  drum  composed  of 
discs  separated  by  some  insulating  material,  and 
provided  with  holes  for  ventilation.  The  armature 
coils  are  formed  of  heavy  conductors  laid  on  the  sur 
face  of  a  drum,  to  which  they  are  securely  attached, 
in  order  to  prevent  their  separation  during  rotation. 

The  number  of  armature  coils  is  equal  to  the 
number  of  field  magnet  poles.  In  some  cases  an  al- 


ALTERNATING  CURRENT  DISTRIBUTION.       217 

ternating  current  dynamo  is  separately  excited;  in 
other  cases  some  of  the  armature  coils  are  suitably 
connected  to  commutators  so  as  to  furnish  the  direct 
currents  that  are  employed  for  the  magnetization  of 
the  field  magnets. 

The  current  produced  in  the  separate  coils  of  the 
armature  may,  before  connection  to  the  collection 
ring,  be  either  connected  with  one  another  in  series 


Fra.  96,-CiRcuiT  CONNECTIONS  OF  ARMATURE  COILS. 

or  they  may  be  connected  in  multiple  arc.  The 
manner  of  their  connection  in  series  will  be  under- 
stood from  an  inspection  of  Fig.  96. 

Since  the  electromotive  forces  employed  in  the 
distribution  of  alternating  currents  by  transformers 
connected  in  multiple  to  constant  potential  leads  are 
considerable,  care  must  be  taken  to  avoid  the  entrance 


218  ELECTRICAL  MEASUREMENTS. 

of  such  high  potential  currents  into  the  buildings 
where  the  low  potential  currents  produced  in  the 
secondary  are  utilized.  For  this  purpose  the  trans- 
formers should  either  be  placed  on  the  outside  of 
the  building  or  on  poles,  where  they  are  out  of  reach 
and  are  thus  placed  out  of  accidental  contact  or  tam- 
pering by  unauthorized  persons,  and  the  secondary 
circuits  be  thoroughly  insulated  from  the  primary. 
Elihu  Thomson  places  a  metallic,  earth-connected 
plate  between  the  primary  and  secondary  circuits  in 
order  to  discharge  the  said  primary  current  to  the 
earth  in  case  of  accidental  contact. 

During  action  a  disagreeable  musical  note,  the 
pitch  of  which  varies  with  the  rapidity  of  the  alterna- 
tion, is  often  emitted  by  the  transformers.  The 
disagreeable  effects  of  this  note  may  be,  to  a  great 
extent,  avoided  by  placing  the  transformers  in  non- 
elastic  connection  with  their  supports. 

When  an  alternating  current  is  sent  through  an  in- 
canijescent  lamp,  although  its  filament  alternately 
increases  and  decreases  in  temperature,  and  the  light 
it  emits  undergoes  similar  changes  in  intensity,  yet 
the  effect  which  such  light  produces  on  the  eye  will 
be  that  of  a  steady,  non-flickering  light,  provided 
the  rate  of  alternation  is  sufficiently  rapid.  A  simi- 
lar effect  is  produced  in  the  Jablochkoff  candle.,  in 


ALTERNATING  CURRENT  DISTRIBUTION.       319 

which  alternating  currents  are  used.  In  order  to 
insure  this  steadiness  in  the  illuminating  power,  the 
alternations  should  not  follow  one  another  too 
slowly.  A  fairly  steady  light  is  obtained  with  as 
few  as  80  reversals  per  second.  A  higher  rate, 
however,  is  preferable. 

In  direct  or  continuous  current  distribution  of 
electricity,  by  means  of  transformers,  a  number  of 
different  systems  have  been  devised.  In  one  of  these 
systems  devices  called  motor-generators  are  employed. 
In  this  system  of  distribution  by  motor-generators  a 
continuous  current  of  high  potential  is  distributed 
through  a  main  line  or  conductor,  and  at  the  points 
where  this  energy  is  to  be  consumed,  the  high 
tension  current  is  utilized  for  driving  a  motor, 
which  in  turn  drives  a  dynamo,  the  current  of  which 
is  of  low  tension  and  great  quantity,  suitably  em- 
ployed for  feeding  the  electro-receptive  devices. 

In  some  cases  the  motor  and  generators  are 
combined  in  a  single  double-wound  armature,  the 
fine  wire  coils  of  which  receive  the  high  potential 
driving  current,  and  the  coarse  wire  coils  furnish 
the  low  potential  currents  used  in  the  distribution 
circuits. 

In  another  system  of  distribution  by  means  of 
continuous  currents,  such  currents  are  sent  over  the 


220  ELECTRICAL  MEASUREMENTS. 

line,  and,  at  the  distant  point  where  it  is  desired  to 
utilize  their  energy,  a  device  called  a  disjunctor  is 
employed  to  rapidly  and  periodically  reverse  their 
direction.  The  alternating  currents  so  obtained  are 
used  either  by  means  of  suitable  transformers  to 
change  the  character  of  the  electromotive  force  and 
the  current  strength  to  meet  the  requirements  of  the 
distribution  circuit,  or  are  employed  directly  to 
charge  condensers  in  the  circuits  of  which  induction 
coils  are  placed.  Sometimes,  however,  the  opposite 
plates  of  the  condensers  are  connected  directly  to 
the  incandescent  electric  lamps. 

When  it  is  desired  to  obtain  a  greater  current 
strength  on  an  alternating  current  circuit  than  can 
be  supplied  by  a  single  dynamo,  it  is  necessary  to 
couple  or  connect  two  such  dynamos  in  multiple  or 
parallel.  To  do  this  it  is  not  necessary  that  the  dyna- 
mos be  first  synchronized;  for,  if  the  armature  circuits 
can  have  their  currents  rapidly  reversed,  and  small 
electro-motive  forces  impressed  •  hereon  produce  large 
currents  and  the  driving  engines  are  not  governed, 
then  the  machines,  even  if  out  of  synchronism  when 
coupled,  will  rapidly  pull  each  other  into  synchronism. 

The  peculiar  effects  of  self-induction  produced  by 
alternating  currents  render  it  possible  to  prevent  va- 
riations in  the  light  emitted  by  a  number  of  incan- 


ALTERNATING  CURRENT  DISTRIBUTION.       221 

descent  lamps  placed  in  parallel  on  any  single  distri- 
bution circuit. 

A  device  frequently  employed  for  such  purposes 
is  called  a  choking,  kicking,  or  reaction  coil.  Such 
coils  are  used  to  obstruct,  choke  or  cut  off  an  alter- 
nating current  with  a  loss  of  power  less  than  by  the 
use  of  a  mere  ohmic  resistance. 

A  choking  coil  is  shown  in  Fig.  97.  It  consists 
of  a  circular  solenoid  of  insulated  wire,  wound  on  a 
core  of  soft  iron  wire,  the  separate  turns  of  which  are 


FIG.  97.— CHOKING-COIL. 

insulated  from  one  another.  By  using  iron  wire  for 
the  core  no  eddy  currents  are  produced  in  the  coil. 

The  higher  the  periodicity  the  greater  is  the 
choking  effect  of  a  given  coil,  or  the  smaller  the 
coil  may  be  made  in  order  to  insure  a  given  effect. 

The  choking  coil  operates  by  generating  a  counter- 
electromotive  force,  which  tends  to  balance  the 
applied  electromotive  force,  and  therefore  cuts  it 
down  or  chokes  it. 

Since  a  magnetic  circuit  completed  partly  through 


222  ELECTRICAL  MEASUREMENTS. 

iron  and  partly  through  air  requires  a  greater  cur- 
rent to  produce  saturation  than  a  closed  magnetic 
circuit,  the  throttling  or  choking  power  of  such  a 
coil  is  increased  by  forming  its  core  in  a  closed  mag- 
netic circuit  ;  that  is,  of  a  circuit  in  which  there  is 
no  air-space  or  gap. 

A  choking  coil  is  sometimes  employed  for  the  pur- 
pose of  varying  the  intensity  of  the  light  emitted  by 
incandescent  lamps  in  a  device  known  as  the  dim 


FIG.  98.— THE  DIMMER. 

The  dimmer  consists,  as  shown  in  Fig.  98,  essen- 
tially of  a  choking  coil  wound  around  a  portion  of  a 
laminated  ring  of  soft  iron.  A  laminated  drum  of 
iron  is  placed  inside  the  ring,  and  suitably  sup- 
ported in  bearings.  There  is  fastened  to  the  drum 
a  heavy  copper  sheath,  which  is  rotated  with  it. 
By  moving  this  sheath  so  as  to  slide  over  and 
cover  more  or  less  of  the  coil,  the  self-induction  in 


ALTERNATING  CURRENT  DISTRIBUTION.       223 

the  coil  becomes  less,  and  therefore  the  current 
which  can  pass  through  it  will  become  greater ; 
when  the  sheath  is  moved  away  from  the  coil,  the 
current  which  can  pass  becomes  less.  The  dimmer  is 
used  in  theatres  or  similar  situations  to  turn  the 
lights  up  or  down,  and  in  central  stations  for  ad- 
justing the  difference  of  potential  of  the  feeders. 

The  balanced  reactive  coil,  an  invention  of  Prof. 
Elihu  Thomson,  is  a  device  for  maintaining  a  constant 
current  in  the  secondary,  and  is  shown  in  Fig.  99.  It 


FIG.  99.— BALANCED  REACTIVE  COIL. 

consists  essentially  of  a  choking  coil,  which  is  so 
counter-balanced  as  to  automatically  adjust  the  po- 
tential in  a  circuit  of  lamps. 

In  this  form  the  coil  is  provided  with  a  metallic 
sheath  which  is  maintained  in  a  balanced  position 
by  the  action  of  a  counter  weight  at  P,  and  the 
spring  S. 

When  a  lamp  is  extinguished    in    the   circuit   the 


224  ELECTRICAL  MEASUREMENTS. 

variation  in  current  due  to  such  variation  in  resistance 
causes  the  sheath  to  be  deflected  and  thus  increases 
the  self-induction  of  the  coil  and  reduces  the  cur- 
rent in  the  lamp  circuit  to  its  normal  value. 


ALTERNATING  CURRENT  DISTRIBUTION.       225 


EXTRACTS  FROM  STANDARD  WORKS. 

The  advantages  to  be  derived  from  the  use  of 
high  potential  alternating  currents  in  systems  of 
distribution,  when  the  electro-receptive  devices  are 
situated  at  comparatively  great  distances  from  the 
source,  is  thus  discussed  by  Urquhart  in  his  "  Elec- 
tric Light,"*  on  page  191. 

In  the  practical  distribution  of  electrical  currents  for 
lighting  it  was  soon  found  that  to  convey  large  currents  at 
low  potential  to  a  distance,  conductors  so  large  as  to  be  im- 
practicable were  required.  On  the  other  hand,  although  it 
was  well  known  that  small  currents  of  high  tension  repre- 
senting the  same  amount  of  energy  could  be  conveyed 
easily  in  exceedingly  small  conductors,  the  principle  could 
not  be  applied  to  ordinary  lamps  direct,  and  the  introduc- 
tion of  such  currents  into  dwellings  would  be  a  possible 
source  of  danger  to  life. 

It  has  long  been  known  that  a  low  tension  current  could 
by  suitable  means  be  converted  into  a  high  tension  current. 

The  induction  coil,  an  instrument  for  this  purpose,  is  too 
well  known  to  call  for  description.  Its  theory  has  been  ex- 
haustively treated  in  most  text-books  of  electricity.  The 
most  powerful  machine  of  this  kind  was  owned  by  the  late 

•"Electric  Light:  Us  Production  and  Use/"  by  John  W.  Urquhart. 
London:  Crosby  Lock  wood  &  Son.  1890.  380  pages,  145  illustra- 
tions. Price  *3.00. 


226  ELECTRICAL  MEASUREMENTS. 

Mr.  Spottiswoode*  F.  R.  S.  This  coil  would  convert  a  low 
tension  and  harmless  current  into  a  high  tension  discharge, 
which  would  flash, across  an  air-space  45  inches  in  width. 
Thus,  from  a  current  of  a  few  volts  a  conversion  was  made 
to  a  current  of  many  million  volts. 

But  the  induction  coil  is  reversible,  for,  by  feeding  its 
secondary  coil  with  a  high  tension  alternating  or  inter- 
rupted cu:  rent,  a  low  tension  current  of  great  volume  is 
obtainable  from  the  primary  coil. 

Thus,  the  induction  coil,  which  until  within  a  few  years 
ago  was  but  a  scientific  toy,  has  developed  into  a  most  im- 
portant auxiliary  to  the  dynamo-electric  machine. 

In  so  far  as  the  use  of  transformers,  as  they  are  generally 
called,  has  taken  effect,  they  have  only  been  successfully 
used  for  alternating  currents.  It  is  well  known  that  if  a 
constant  current  be  passed  through  the  primary  wire  of  an 
induction  coil  the  secondary  circuit  will  evince  no  sign  of 
current.  At  the  moment  of  making  or  breaking  contact 
with  the  primary  coil,  however,  momentary  currents  will 
flow  in  the  secondary  coils.  Hence  the  necessity  to  use  a 
contact  breaker  or  interrupter  with  such  coils. 

But  if  a  constant  current  flows  in  the  primary  coil,  and 
that  coil  be  moved  within  the  secondary  coil,  currents  cor- 
responding to  the  motions  will  be  induced  in  the  secondary 
— in  fact,  we  have  now  a  kind  of  dynamo  machine.  Hence, 
if  the  current  transformer  can  be  used  for  converting  con- 
stant currents  of  high  force  to  constant  currents  of  low 
force,  they  must  take  the  form  of  machines  of  some  kind. 


XL— ELECTRIC  CURRENTS  OF  HIGH  FRE- 
QUENCY. 


When,  in  the  case  of  alternating  currents,  the 
rate  of  alternation  and  the  electromotive  force  be- 
come very  high,  a  number  of  phenomena  present 
themselves  that  differ  in  many  respects  from  those 
of  alternating  currents  of  only  moderately  high  fre- 
quency. Some  of  these  differences  may  be  noticed 
as  follows : 

(1.)  When  alternating  currents  of  very  small  fre- 
quency are  sent  through  an  electro-magnet,  and  a 
piece  of  iron  is  held  against  its  poles,  the  alternate 
attractions  and  repulsions  of  its  armature  can  be 
readily  felt  by  the  hand  ;  if  the  frequency  of  the  al- 
ternations is  increased,  the  impulses  felt  follow  one 
another  faster,  but  become  weaker,  and,  when  the 
frequency  becomes  much  higher,  there  is  apparently 
nothing  but  attraction,  which  manifests  itself  by  a 
continuous  pull. 

(2.)  Many  substances  which  act  as  insulators  for 
steady  currents  will  permit  alternating  currents  to 

readily  pass  through  them,  provided    the  frequency 
(227) 


228  ELECTRICAL  MEASUREMENTS. 

and  difference  of  potential  are  sufficiently  high. 
This  is  especially  the  case  if  a  gas  be  present. 
The  discharge  will  then  take  place  by  molecular 
bombardment  through  considerable  thicknesses  of 
such  insulators  as  glass,  hard  rubber, sealing  wax,  etc. 

Therefore,  in  the  insulation  of  conductors  con- 
veying discharges  of  very  high  frequencies,  where 
the  differences  of  potential  are  great,  all  air  and  gas 
must  be  carefully  excluded. 

(3.)  Conducting  substances  which  readily  permit 
the  passage  of  steady  currents  offer  a  resistance  to 
alternating  currents  which  increases  in  amount  as  the 
frequency  of  the  alternations  increases,  until  at  last, 
when  the  frequency  of  alternation  has  reached 
certain  high  limits,  such  substances  act  practically 
as  non-conductors. 

(4.)  The  physiological  effects  of  alternating  cur- 
rents of  but  moderate  frequency  are  unquestionably 
severe.  When,  however,  the  rapidity  of  alternation 
increases,  alternating  currents  undoubtedly  become 
less  severe  in  their  physiological  action,  and  this 
decrease  becomes  the  more  marked  as  the  frequency 
of  alternation  increases,  until  finally  they  appar- 
ently become  absolutely  harmless. 

This  is  probably  due  to  the  fact  that,  when  such 
increase  of  rapidity  of  alternation  is  sufficiently  great, 


ELECTRIC  CURRENTS  OF  HIGH  FREQUENCY.    229 

only  the  superficial  portions  of  the  body  are  af- 
fected, the  increase  in  rapidity  of  alternation  pro- 
ducing a  counter-electromotive  force  or  spurious  re- 
sistance, which  opposes  the  passage  of  the  current 
through  the  body. 

Nikola  Tesla  was  the  first  to  make  extended  in- 
vestigations in  the  field  of  alternating  currents  of 
exceedingly  high  frequency,  and  it  is  to  him  that 
the  credit  is  due  for  most  of  the  above-mentioned 
facts  concerning  the  difference  in  the  behavior  of 
alternating  currents  of  exceedingly  high  and  of  but 
moderately  high  frequency. 

Tesla  obtained  the  exceedingly  high  frequencies 
with  which  he  experimented  in  the  following  man- 
ner :  By  means  of  a  multipolar  dynamo-electric 
machine,  the  armature  of  which  was  driven  at  a 
high  peripheral  speed,  and  the  currents  of  which 
were  uncommuted,  he  obtained  alternating  cur- 
rents of  very  high  frequency.  These  currents  were 
sent  through  the  primary  circuit  of  a  peculiarly  con- 
structed induction  coil.  Under  these  circumstances 
Tesla  noticed  a  number  of  exceedingly  peculiar 
effects  in  the  luminous  character  of  the  discharge 
taken  between  the  secondaries  of  the  induction  coil. 

Tesla  describes  five  distinct  characters  of  high  fre- 
quency luminous  discharge  ;  namely  : 


230  ELECTRICAL  MEASUREMENTS. 

(1.)  The  sensitive-thread  discharge,  in  which  a 
thin,  thread-like  discharge  occurs  between  the  ter- 
minals of  the  secondary  of  the  induction  coil  as  soon 
as  a  certain  high  frequency  is  reached. 

According  to  Tesla,  this  discharge  occurs  when 
the  number  of  alternations  in  the  primary  is  high, 
and  the  strength  of  the  current  passing  is  small. 
The  discharges  present  the  appearance  of  thin, 
feebly  colored  threads.  Though  easily  deflected  by 


FIG.  IOO.-SENSITIVE-THREAD  DISCHARGE  (TESLA). 

the  breath,  such  discharge  is  quite  persistent,  re- 
sisting efforts  to  blow  it  out,  provided  the  terminals 
are  placed  at  one-third  the  striking  distance  apart.. 
Tesla  ascribes  its  extreme  sensitiveness,  when  long, 
to  the  motion  of  dust  particles  suspended  in  the 
air  through  which  the  spark  is  passing.  The  general 
appearance  of  the  sensitive-thread  discharge  is  shown 
in  Fig.  100. 


ELECTRIC  CURRENTS  OF  HIGH  FREQUENCY.    231 

(2.)  The.flaming  discharge,  which  takes  place  when 
the  current  through  the  primary  of  the  induction 
coil  is  increased.  Under  these  circumstances  the 
thickness  of  the  discharge  increases  until  it  assumes 
a  naming  white,  arc-like  discharge. 

According  to  Tesla,  the  naming  discharge  occurs 
when  the  number  of  alternations  per  second  is  great, 
and  the  other  conditions  of  the  circuit  are  such  as 
will  permit  the  passage  through  the  primary  of  the 
coil  of  the  maximum  current. 


FIG.  101.— FLAMING  DISCHARGE  (TESLA). 

The  flaming  discharge  develops  considerable  heat. 
The  shrill  note  that  accompanies  less  powerful  dis- 
charges is  absent  in  the  flaming  discharge.  This, 
most  probably,  results  from  the  enormous  frequency 
of  the  discharge  carrying  the  note  far  above  the 
limits  of  audition. 

Some  idea  of  the  general  appearance  of  .the  flam- 
ing discharge  may  be  had  from  an  inspection  of  Fig. 


232  ELECTRICAL  MEASUREMENTS. 

101.  According  to  Tesla,  the  conditions  for  its  pro- 
duction, in  an  ordinary  induction  coil  of  say  10,000 
ohms  resistance,  is  best  obtained  by  a  rate  of  alter- 
nation of  about  12,000  per  second. 

(3.)  The  streaming  discharge,  which  occurs  as  the 
frequency  of  the  discharge  through  the  secondary 
increases  and  the  current  strength  decreases.  The 
streaming  discharge  partakes  of  the  general  nature 
of  the  flaming  discharge.  Luminous  streams  pass 


FIG.  102.— STREAMING  DISCHARGE  (TESLA). 

in  abundance,  not  only  between  the  terminals  of  the 
secondary,  but  even  between  the  primary  and  the  sec- 
ondary through  the  insulating  dielectric  substances 
separating  them.  They  also  issue  from  all  points 
and  projections,  as  shown  in  Fig.  102. 

(4.)  The  brusli-and-spray  discharge,  which  occurs 
when  the  streaming  discharge  reaches  a  certain 
higher  limit. 

The   streaming  discharge   that  can   be  obtained 


ELECTRIC  CURRENTS  OF  HIGH  FREQUENCY.    233 

from  an  induction  coil  with  high  frequencies  differs 
from  that  obtained  from  an  electrostatic  machine  in 
that  it  neither  possesses  the  violet  color  of  the  posi- 
tive discharge  nor  the  brightness  of  the  negative 
discharge,  but  is  paler  in  color. 

The  appearance  of  the  brnsh-and-spray  discharge 
is  shown  in  Fig.  103. 

The  brush-and-spray  discharge,  when  powerful, 
closely  resembles  a  gas  flame  issuing  from  a  gas 


FIG.  103.— BRUSH-AND-SPRAY  DISCHARGE  ( 
burner  under  great  pressure.     Speaking  of  such  dis- 
charges Tesla  says  : 

"  But  they  do  not  only  resemble,  they  are  veritable 
flames,  for  they  are  hot.  Certainly  they  are  not  as 
hot  as  a  gas  burner,  but  they  would  be  so  if  the  fre- 
quency and  the  potential  would  be  sufficiently  high." 

(5.)  Tesla' s  fifth  form  of  discharge.  This  form  of 
discharge  occurs  as  a  change  of  the  brush-and-spray 
discharge  when  the  frequency  of  the  discharge 


234  ELECTRICAL  MEASUREMENTS. 

through  the  primary  is  still  further  increased.  The 
terminals  now  refuse  to  give  sparks  except  at  very 
short  distances,  probably  from  the  exaggerated  ten- 
dency to  dissipate.  At  this  stage  the  discharge  ap- 
pears to  pass  through  thick  insulators  with  great 
readiness.  The  appearance  of  this  form  of  discharge 
is  shown  in  Fig.  104. 

Beautiful  luminous  phenomena  are  produced  by 
interposing  or  placing  insulating  substances  between 


FIG  104.— FIFTH  TYPICAL  FORM  OP  DISCHARGE  (TESLA). 

the  terminals  of  a  coil  arranged  so  that  this  fifth  form 
of  discharge  has  been  obtained.  If,  for  example, 
a  thin  plate  of  ebonite  is  placed  between  the  ter- 
minals, each  of  which  has  been  provided  with  me- 
tallic spheres,  as  shown  in  Fig.  105,  the  sparks 
cease,  and,  provided  the  spheres  are  sufficiently 
large,  the  discharge  produces  instead  an  intensely 
luminous  circle  several  inches  in  diameter. 

Under  certain  circumstances  the  brush  discharge 


ELECTRIC  CURRENTS  OF  HIGH  FREQUENCY.    235 

may  be  obtained  in  a  very  powerful  form.  This  is 
best  obtained  when  the  terminals  of  the  secondary 
are  attached  to  bodies  whose  surfaces  can  be  adjusted 
to  give  the  best  results  with  that  particular  coil  and 
that  particular  discharge.  In  this  case  the  brush 
discharge  becomes  very  marked,  and  gives  off 
streams  of  intensely  heated  air  particles. 

Tesla  proposes  the  name  of  hot  St.  Elmo's  fire  far 


FIG.  105.— LUMINOUS  DISCHARGE  WITH  INTERPOSED  INSULATORS. 

this  form  of  discharge,  which   has  the  appearance 
shown  in  Fig.  106. 

Since  current  impulses  produced  by  alternating 
discharges  of  such  high  frequency  cannot  be  passed 
through  conductors  of  measurable  dimensions,  Tesla 
has  experimented  as  to  the  effects  which  such  dis- 
charges produce  on  refractory  insulating  materials 
placed  inside  of  closed  vessels  by  subjecting  such 


236  ELECTRICAL  MEASUREMENTS. 

substances  to  the  thrusts  of  alternating  electrostatic 
fields.  By  connecting  vessels  or  globes  with  sources 
of  rapidly  alternating  potential,  under  the  influence 
of  the  alternating  electrostatic  thrusts,  the  molecules 
of  the  residual  gas  are  set  into  motion  with  enor- 
mous velocity,  and  the  molecular  shocks  thus  given 
to  the  refractory  material  render  it  highly  luminous. 
These  effects  are  best  obtained  in  vacuous  spaces. 


FIG.  106.— HOT  ST.  ELMO'S  FIRE. 

In  obtaining  the  effects  of  luminosity  by  the  bom- 
bardment  of  the  molecules  of  the  residual  gas  it  is 
not  necessary  to  connect  both  terminals  of  the  vessel 
to  the  source  ;  a  single  connection  suffices.  In  this 
manner  a  new  species  of  electric  lamp  is  produced, 
which  may  be  called  the  electric  bombardment  lamp. 

Tesla  has  constructed  a  great  variety  of  electric 
bombardment  lamps,  in  which  single  or  double  fila- 


ELECTRIC  CURRENTS  OF  HIGH  FREQUENCY.    237 

ments  are  employed  connected  to  either  one  or  both 
terminals  of  a  rapidly  alternating  source. 

When  an  electric  bombardment  lamp  is  operated 
by  connecting  it  to  a  single  terminal  only,  its  brill- 
iancy is  greatly  increased  by  connecting  one  terminal 
to  a  conducting  surface  placed  on  the  outside  of  the 
lamp  and  acting  as  a  condenser  coating,  and  connect- 
ing the  other  terminal  to  a  solid  body  of  the  same 
extent  of  surface  as  the  condenser  coating. 

When  electric  bombardment  lamps  are  provided 
with  an  external  coating,  they  can  be  lighted  when 
suspended  from  a  single  terminal  in  any  part  of  the 
room.  In  some  cases  Tesla  succeeded  in  lighting  a 
lamp  by  placing  it  anywhere  in  the  rapidly  alter- 
nating electrostatic  field  obtained  between  two  ex- 
tended plates  of  metal.  Here  no  connection  of  either 
terminal  to  the  lamp  is  necessary,  and  the  lamp  re- 
mains lighted  as  it  is  carried  about  in  different  parts 
of  the  room. 

In  order  to  obtain  extraordinarily  high  frequencies, 
Tesla  adopted  the  plan  of  employing  the  discharges 
obtained  as  above  from  the  secondary  of  the  induc- 
tion coil  to  charge  a  condenser,  the  discharges  of 
which  were  employed  to  feed  the  primary  of  a  sec- 
ond induction  coil,  the  secondary  of  which  was  con- 
nected to  the  circuit  of  the  lamps  to  be  lighted. 


ELECTRICAL  MEASUREMENTS. 


The  general  arrangement  of  the  apparatus  em- 
ployed by  Tesla  for  this  purpose  is  shown  in  Fig. 
107.  G,  is  a  dynamo  producing  alternating  currents 
of  comparatively  low  potential  but  high  frequency. 

In  this  circuit  is  placed  the  primary  coil  P,  of 
an  induction  coil,  which  induces  alternating  currents 
of  high  potential  in  the  secondary  circuit  S. 

These  currents  are  employed  for  charging  the  con- 
denser C,  which  is  provided  with  an  air  gap  at  A,  and 


MMAMA 


A&AWMM 


FIG. 


7.— TKSLA'S  HIGH-FREQUENCY  CURRENTS  SYSTEM  OF 
LIGHTING. 

another  primary  coil  P'.  The  discharges  of  the  con- 
denser across  the  air  gap  produce  oscillatory  or 
alternating  currents  of  enormous  frequency,  which 
in  passing  through  the  primary  P'}  produce  similar 
currents,  but  of  very  high  potential,  in  the  secondary 
coil  S'. 

Two  incandescent  electric  lamps,  as  shown  in  the 


ELECTRIC  CURRENTS  OF  HIGH  FREQUENCY.    239 

figure,  are  connected  to  one  pole  of  the  secondary 
circuit ;  one,  an  incandescent-ball  lamp,  and  the 
other  a  single  straight-filament  lamp.  The  other 
pole  of  the  secondary  circuit  is  connected  to  a  large 
plate  W  W.  The  construction  of  these  two  lamps  is 
shown  respectively  in  Figs.  108  and  109. 

In  Tesla's  incandescent-ball  electric  bombardment 
lamp,  electrostatic  waves  of  high  frequency  of  alter- 


FIG.  108.— TESLA'S  INCANDESCENT-BALL  ELECTRIC  LAMP. 
nation,  acting  on  a  sphere  or  ball  of  carbon  connected 
with  a  single  filament,  as  shown  in  Fig.  108,  and 
placed  inside  the  vacuous  space  of  a  glass  chamber, 
render  such  ball  or  sphere  incandescent. 

In  Tesla's  straight  filament  incandescent  lamp,  the 
carbon  ball  is  replaced  by  a  straight  filament  of 
carbon  placed  inside  an  exhausted  glass  chamber.  The 
filament  is  rendered  highly  luminous  on  being  ex- 


240  ELECTRICAL  MEASUREMENTS. 

posed  to  electrostatic  thrusts  or  waves  of  high  fre- 
quency. 

The  glass  globe  b,  Fig.  109,  of  the  lamp,  is  pro- 
vided with  a  cylindrical  neck,  inside  of  which  is 
placed  a  tube  m,  of  conducting  material,  on  the  side 
and  over  the  end  of  the  insulated  plug  n. 


FIG.  109.— TESLA'S  STRAIGHT-FILAMENT  INCANDESCENT  LAMP. 
The  light-giving  filament  e,  is  a  straight  carbon 
stem,  connected  to  the  plate  by  a  conductor  covered 
with  a  refractory,  insulating  material  k.  An  insu- 
lated tube-socket^,  provided  with  a  metallic  lining 
s,  serves  to  support  the  lamp  and  connect  it  with 


ELECTRIC  CURRENTS  OF  HIGH  FREQUENCY.    fi4l 

one  pole  -of  the  source  of  alternating  discharges. 
The  other  terminal  of  the  machine  may  be  connected 
to  the  metal-coated  walls  of  the  room,  or  to  metallic 
plates  suspended  from  the  ceiling. 


FIG.  110.— DISRUPTIVE  DISCHARGE  Coit. 

When  condensers  are  employed  to  charge  the  prima- 
ry of  a  transformer.  Tesla  employed  for  certain  exper- 
iments the  transformer  shown  in  Fig.  110,  in  which 


243  ELECTRICAL  MEASUREMENTS, 

the  box  B,  of  hard  wood,  is  covered  on  the  outside 
with  zinc.  The  coils  consist  of  spools  of  hard  rub- 
ber wound  with  gutta-percha  covered  wires,  P  P} 
and  8  S,  which  form  the  primaries  and  secondarin 
respectively.  In  order  to  avoid  the  effects  of  the 


FIG.  111.— DIRECTED  STREAMING  DISCHARGE. 
metal  covering  of  the  box  the  coils  are  placed  as  near- 
ly at  the  centre  of  the  box  as  possible.  Both  the 
primary  and  secondary  coils  are  wound  on  the  spools 
in  two  equal  parts.  The  box  is  filled  with  oil  from 
which  all  air  or  gas  has  been  removed  by  boiling. 


ELECTRIC  CURRENTS  OF  HIGH  FREQUENCY.    243 

When  the  conditions  are  such  in  the  operation  of 
such  a  coil  that  a  streaming  discharge  has  been  ob- 
tained, by  connecting  the  terminals,  as  shown  in 
Fig.  Ill,  a  hollow  continuous  luminous  cone  is  ob- 
tained between  the  electrodes  W  and  $. 


FIG.  112.— LUMINOUS  DISC-SHAPED  DISCHARGE. 

In  a  similar  manner,  if  the  terminals  be  shaped  in 
the  form  of  rings  that  are  placed  consecutively  in  as 
nearly  the  same  plane  as  possible,  as  shown  in  Fig. 


244 


ELECTRICAL  MEASUREMENTS. 


112,  the  entire  space  between  the  concentric  elec- 
trode is  filled  by  a  luminous  discharge. 

Probably  the  most  curious  form  of  discharge  ob- 
tained by  Tesla  among  tbe  many  other  remarkable 


FIG.  113.— ROTATING-BRUSH  DISCHARGE. 

forms  is  what  he  calls  the  rotating-brush  discharge, 
and  is  shown  in  Pig.  113. 

This  discharge  is  taken  in  a  lamp  chamber  or  bulb 


ELECTRIC  CURRENTS  OF  HIGH  FREQUENCY.    ;  45 

containing  a  high  vacuum.  A  barometer  tube  I, 
Fig.  114,  is  placed  inside  the  chamber  and  is  blown 
out  into  a  small  bulb,  s,  at  one  end,  and  is  placed  as 
shown  in  Figs.  113  and  114.  Under  certain  con- 


Fio.  114.— APPARATUS  FOR  ROTATINO-BRUSH  DISCHARGE. 

ditions  a  brush  discharge  is  obtained  that  is  ex- 
ceedingly sensitive  to  electrostatic  or  magnetic  influ- 
ences. If  the  bulb  is  attached  to  a  single  terminal 


246  ELECTRICAL  MEASUREMENTS, 

at  its  lower  end,  and  is  hanging  vertically  downward, 
the  mere  approach  of  an  observer  will  cause  the  brush 
to  fly  to  the  opposite  side,  and,  if  he  walks  around 
the  bulb,  it  will  move  with  him,  always  keeping,  on 
the  opposite  side.  In  the  Northern  Hemisphere  the 
rotation  is  invariably  clock-wise. 

Elihu  Thomson  obtains  discharges  of  high  fre- 
quency and  enormous  difference  of  potential  by  em- 
ploying discharges  of  high  potential  from  a  con- 
denser, to  produce  electro-dynamic  induction  in 
induction  coils.  The  high  insulation  necessary  for 
separating  the  primary  from  the  secondary  coils  in 
such  induction  apparatus  is  obtained  by  surround- 
ing them  with  oil.  By  these  means  he  obtains  dis- 
charges through  thirty  inches  of  air  at  differences  of 
potential  that  have  been  estimated  to  be  as  high  as 
500,000  volts. 

In  his  apparatus  Thomson  employs  Ley  den  jars 
as  condensers.  The  circuit  connections  between  the 
jar  and  the  induction  coil  will  be  understood  from 
an  inspection  of  Fig.  115  and  the  following  descrip- 
tion. 

The  construction  of  the  apparatus  employed  by 
Thomson  is  described  by  him  as  follows  : 

"  A  double  coil  was  made,  of  which  the  inner 
turns  were  about  twelve  and  the  outer  turns  twenty. 


ELECTRIC  CURRENTS  OF  HIGH  FREQUENCY.    247 

These  were  kept  separated  from  each  other,  and  a 
branch  wire  taken  from  the  line  and  slid  from  point  to 
point  on  the  outer  wire  enabled  the  effective  length 
of  the  sameto.be  adjusted.  The  inner  coil  was  con- 
nected through  a  small  spark  gap,  as  at  A,  to  the 
outer  coating  of  a  Leyden  jar,  while  the  wire  L,  was 
brought  near  the  pole  of  the  jar,  which  was  continu- 
ously being  charged  from  a  Topler-Holtz  machine 


O.QO 


FIG.  115.— THOMSON'S  APPARATUS. 

the  discharge,  in  passing  from  the  knob  of  the  jar  to 
the  wire  L,  representing  the  line  passed  by  the  inner 
coil.  When  a  certain  length  of  the  outer  coil  was 
employed  only  a  very  short,  almost  imperceptible, 
spark  was  obtained  at  A.  If  the  balance  of  the  turns 
were  disturbed  by  including  more  or  less  than  the 
proper  number  of  the  outer  turns,  not  only  did  a 
vigorous  spark  occur,  but  the  gap  at  A  could  be 


248  ELECTRICAL  MEASUREMENTS. 

quite  considerably  extended,  in  accordance  with  the 
amount  of  departure  taken  from  the  proper  number 
of  turns  required  to  produce  the  balance." 

"When  the  apparatus  is  arranged  as  shown  in  Fig. 
116,  curious  three-branched  sparks  are  obtained, 
which,  under  certain  circumstances,  assume  remark- 

QPO 


FIG.  116.— ARRANGEMENT  OF  APPARATUS  FOR  THREE-BRANCHED 
SPARKS. 

able  T  and  Y  shapes.     One  of  these  shapes  is  shown 
in  Fig.  117. 

As  regards  the  physiological  effects  of  shocks  pro- 
duced by  means  o*f  alternating  currents,  it  can  readily 
be  shown  that  up  to  a  certain  rate  of  frequency 
these  shocks  are  more  severe  and  painful  in  the  case 


ELECTRIC  CURRENTS  Of  HIGH  FREQUENCY.    249 

of  alternating  discharges  than  in  the  case  of  steady 
currents  of  equal  volume  and  potential  difference. 
When,  however,  the  rate  of  alternation  increases 
and  reaches  a  certain  limit,  the  effects  of  the  alter- 
nating discharge  are  much  less  severe  than  those  of 
a  steady  current. 

Tesla  has  shown,  beyond  any  doubt,  that  when  the 
rapidity  of  alternation  increases  beyond  a  certain 
extent,  the  rapidly  alternating  currents  produced 
are  much  less  dangerous  in  their  effects  than  a  low 


FIG.  117.— THREE-BRANCHED  SPARKS. 

frequency  discharge  of  the  same  difference  of  po- 
tential. Speaking  of  this  fact  before  the  American 
Institute  of  Electrical  Engineers  on  the  20th  of 
May,  1891,  Tesla  says  : 

"I  have  found  that  by  using  the  ordinary  low 
frequencies  the  physiological  effect  of  the  current 
required  to  maintain  at  a  certain  degree  of  bright- 
ness a  tube  four  feet  long,  provided  at  the  ends 
with  outside  and  inside  condenser  coatings,  is  so 
powerful  that  I  think  it  might  produce  serious  in- 


250  ELECTRICAL  MEASUREMENTS. 

jury  to  those  not  accustomed  to  such  shocks ; 
whereas  with  20,000  alternations  per  second  the 
tub'e  may  be  maintained  at  the  same  degree  of  bright- 
ness without  any  effect  being  felt. 

"  This  is  due  principally  to  the  fact  that  a  much 
smaller  potential  is  required  to  produce  the  same 
light  effect,  and  also  to  the  higher  efficiency  in  the 
light  production." 

Dr.  Tatum  has  made  a  number  of  experiments  in 
this  direction,  and  reaches  practically  similar  con- 
clusions. 


ELECTRIC  CURRENTS  OF  HIGH  FREQUENCY.    251 


EXTRACTS  FROM  STANDARD   WORKS. 

In  "  Modern  Views  of  Electricity/'  *  Lodge,  on 
page  284,  says  concerning  the  manufacture  of 
light : 

The  conclusions  at  which  we  arrived,  that  light  is  an 
electrical  disturbance,  and  that  light- waves  are  excited  by 
electric  oscillations,  must  ultimately,  and  may  shortly, 
have  a  practical  import. 

Our  present  systems  of  making  light  artificially  are 
wasteful  and  ineffective.  We  want  a  certain  range  of 
oscillation,  between  seven  thousand  billion  and  four 
thousand  billion  vibrations  per  second.  No  other  is 
useful  to  us,  because  no  other  has  any  effect  on  our 
retina ;  but  we  do  not  know  how  to  produce  vibrations 
of  this  rate.  We  can  produce  a  definite  vibration  of  one  or 
two  hundred  or  thousand  per  second  ;  in  other  words,  we 
can  excite  a  pure  tone  of  definite  pitch,  and  we  can  com- 
mand any  desired  range  of  such  tones  continuously  by 
means  of  bellows  and  a  keyboard.  We  can  also  (though 
the  fact  is  less  well  known)  excite  momentarily  definite 
ethereal  vibrations  of  some  million  per  second,  as  I  have 
explained  at  length  ;  but  we  do  not  at  present  seem  to  know 
how  to  maintain  this  rate  quite  continuously.  To  get  much 
faster  rates  of  vibration  than  this  we  have  to  fall  back  upon 

*  "Modern  Views  of  Electricity,"  by  Oliver  J.  Lodge.  IX  Sc., 
LL.  D.,  F.  R.  S.  London:  Macmillan  &  Co.  1889.  480  pages,  67 
illustrations.  Price,  $2. 


252  ELECTRICAL  MEASUREMENTS. 

atoms.  We  know  how  to  make  atoms  vibrate.  It  is  done 
by  what  we  Call  "  heating  "  the  substance,  and  if  we  could 
deal  with  individual  atoms  unhampered  by  others  it  is  possi- 
ble that  we  might  get  a  pure  and  simple  mode  of  vibration 
from  them.  It  is  possible,  but  unlikely  ;  for  atoms,  even 
when  isolated,  have  a  multitude  of  modes  of  vibration 
special  to  themselves,  of  which  only  a  few  are  of  practical 
use  to  us,  and  we  do  not  know  how  to  excite  some  without 
also  the  others.  However,  we  do  not  at  present  even  deal 
with  individual  atoms.  We  treat  them  crowded  together  in 
a  compact  mass,  so  that  their  modes  of  vibration  are  really 
infinite. 

We  take  a  lump  of  matter,  say  a  carbon  filament  or  a 
piece  of  quicklime,  and  by  raising  its  temperature  we  im- 
press upon  its  atoms  higher  and  higher  modes  of  vibration, 
not  transmuting  the  lower  into  the  higher,  but  superposing 
the  higher  upon  the  lower,  until  at  length  we  get  such 
rates  of  vibration  as  our  retina  is  constructed  for,  and  we 
are  satisfied.  But  how  wasteful  and  indirect  and  empirical 
is  the  process.  We  want  a  small  r  inge  of  rupid  vibrations, 
and  we  know  no  better  than  to  m.ike  the  whole  series  lead- 
ing up  to  them.  It  is  as  though,  in  orJer  to  sound  some 
little  shrill  octave  of  pipes  in  an  or^aa,  we  were  obliged  to 
depress  every  key  and  every  pedal,  and  to  blow  a  young 
hurricane. 


XIL— ELECTRO-DYNAMIC  INDUCTION. 


The  general  principles  according  to  which  elec- 
tricity is  produced  by  the  aid  of  magnets  was  dis- 
covered by  Faraday  in  1831. 

In  order  to  obtain  electricity  by  tbe  aid  of  a  mag- 
net it  is  only  necessary  that  a  conductor  be  so 
brought  into  or  moved  through  the  field  of  the  mag- 
net as  either  to  cut  or  to  be  cut  by  its  lines  of  force. 

When  a  conductor  cuts,  or  is  cut  by,  such  lines 
of  force,  differences  of  potential  are  produced  in  it, 
and,  if  such  points  or  places  of  difference  of  poten- 
tial are  connected  by  a  conductor,  so  as  to  complete 
a  circuit,  electric  currents  are  produced. 

The  production  of  electromotive  force  in  this 
manner  by  cutting  lines  of  magnetic  force  is  called 
electro-dynamic  induction. 

"We  have  already  seen  that  when  a  current  of  elec- 
tricity flows  through  a  conductor,  a  magnetic  field, 
traversed  by  lines  of  magnetic  force,  is  produced  in 
the  space  around  the  conductor.  Such  a  magnetic 
field,  like  the  field  produced  by  a  magnet,  can  be 
employed  for  the  production  of  electro-dynamic  in- 
duction. 

When  an  electric   current   passes  through  a  con- 
(253) 


254  ELECTRICAL  MEASUREMENTS. 

ductor  lines  of  magnetic  force  are  produced  in  the 
space  around  the  conductor.  As  the  strength  of  the 
current  through  the  conductor  increases,  the  lines  of 
force  of  the  field  increase  in  number  and  expand  or 
move  outward  from  the  conductor.  As  the  strength 
of  the  current  decreases,  the  lines  of  force  decrease  in 
number  and  contract  or  move  inward  toward  the 
conductor. 

Even  if  a  conductor  be  fixed  it  will  have  differ- 
ences of  potential  induced  in  it  if  it  is  placed  in  the 
neighborhood  of  another  conductor,  through  which 
a  current  is  passing  that  is  rapidly  undergoing 
changes  in  its  strength  ;  for,  the  contracting  and  ex- 
panding lines  of  force  of  the  latter  will  cut  the 
neighboring  conductor  and  produce  currents  in  it  in 
one  direction  as  they  pass  through  it  oh  expanding, 
and  in  the  opposite  direction  as  they  pass  through 
it  on  contracting. 

Electro-dynamic  induction  may,  therefore,  be 
produced  in  two  ways  : 

(1.)  By  moving  a  conductor  across  lines  of 
magnetic  force  so  as  to  cut  these  lines  ;  or, 

(2.)  By  causing  expanding  or  contracting  lines  of 
force  to  pass  across  a  stationary  conductor. 

Four  cases  of  electro-dynamic  induction  may 
arise;  namely, 


ELECTRO-DYNAMIC  INDUCTION.  255 

(1.)  Self-induction  or  inductance;  or  that  form  of 
electro-dynamic  induction  in  which  the  expanding 
and  contracting  lines  of  magnetic  force,  produced 
by  varying  the  strength  of  the  current  in  any  circuit, 
are  caused  to  pass  across  or  cut  that  circuit  and  thus 
produce  differences  of  potential  therein.  Self-induc- 
tion occurs  especially  in  coils. 

(2.)  Mutual  induction  or  voltaic  current  induc- 
tion; or  that  form  of  electro-dynamic  induction  in 
which  the  contracting  and  expanding  lines  of  mag- 
netic force,  produced  in  one  circuit  by  varying  the 
current  strength  in  a  neighboring  circuit,  are  caused 
to  pass  across  another  neighboring  circuit  and  thus 
produce  differences  of  potential  therein. 

(3.)  Magneto-electric  induction  ;  or  that  form  of 
electro-dynamic  induction  in  which  a  conductor  is 
so  moved  across  the  field  of  a  permanent  magnet  as 
to  cut  its  lines  of  magnetic  force  ;  or,  what  is  the 
same  thing,  in  which  a  permanent  magnet  is  moved 
past  a  conductor  so  as  to  cause  its  lines  of  magnetic 
force  to  pass  across  the  conductor  and  thus,  in  either 
case,  to  produce  differences  of  potential  in  the  con- 
ductor. 

(4.)  Electro-magnetic  induction,  or  that  form  of 
electro-dynamic  induction  in  which  a  conductor  is  so 
moved  through  the  field  of  an  electro-magnet,  or  in 


256  ELEQTRICAL  MEASUREMENTS. 

which  an  electro-magnet  is  moved  past  a  conductor, 
so  as  to  ensure  a  cutting  of  its  lines  of  magnetic  force, 
and  thus  produce  differences  of  potential  therein. 

Magneto-electric  and  electro-magnetic  induction 
are  in  reality  one  and  the  same  variety  of  electro- 
dynamic  induction.  They  are,  therefore,  sometimes 
called  dynamo-electric  induction. 

Self-induction. — When  the  circuit  of  a  single  vol- 
taic cell  is  closed  by  connecting  its  terminals  together 
without  including  anything  else  in  the  circuit,  no 
very  intense  spark  is  observed,  either  when  such 
terminals  come  into  contact,  or  when  such  contact 
is  broken.  If,  however,  the  terminals  are  connected 
to  a  comparatively  long  coil  of  insulated  wire, 
although  no  appreciable  spark  is  observed  on  making 
or  closing  the  circuit,  quite  a  considerable  spark  is 
seen  on  breaking  or  opening  the  circuit. 

The  cause  of  the  increased  length  of  spark  thus 
produced,  on  opening  or  breaking  the  circuit  of  the 
cell,  is  as  follows  : 

When,  on  the  closing  of  the  circuit,  the  current 
strength  is  increasing  from  zero,  or  no  strength,  to 
the  full  strength  the  cell  is  able  to  produce,  the 
magnetic  field  of  the  coil  is  increasing,  and  its  lines 
of  magnetic  force  are  expanding  or  moving  out- 
ward. 


ELKCTRO-DYNAMIC  INDUCTION.  257 

When,  on  the  breaking  or  opening  of  the  cir- 
cuit, the  current  strength  in  the  circuit  is  decreas- 
ing, the  lines  of  magnetic  force  are  contracting  or 
moving  inward  toward  the  conductor. 

Now,  the  lines  of  force  moving  either  from  or 
toward  any  portion  of  the  circuit  or  conductor,  on 
expanding  or  contracting  will  pass  through  or  cut 
some  other  portion  of  the  wire,  and  will  thereby 
•produce  differences  of  potential  therein. 

As  soon  as  the  current  strength  in  the  conductor 
becomes  constant  its  lines  of  magnetic  force  no 
longer  move  inward  or  outward,  and  no  effects  of 
electro-dynamic  induction  are  produced.  It  is,  there- 
fore, necessary  in  these  varieties  of  electro-dynamic 
induction  to  rapidly  make  and  break  the  circuit. 

AVhen  the  circuit  is  closed,  during  the  time  the 
current  strength  is  increasing,  the  induced  current 
tends  to  flow  in  a  direction  opposite  to  that  of  the 
inducing  current.  When  the  circuit  is  opened,  dur- 
ing the  time  the  current  strength  is  decreasing, 
the  induced  current  tends  to  flow  in  the  same  direc- 
tion as  the  inducing  current. 

The  two  currents  produced  respectively  on  the 
making  and  the  breaking  of  the  circuit  are  called 
extra  currents.  That  produced  in  making  the  circuit 
is  called  the  extra  inverse  current,  because  it  flows  in 


258  ELECTRICAL  MEASUREMENTS. 

the  opposite  direction  to  the  inducing  current ;  and 
that  produced  on  breaking  the  circuit  is  called  the 
extra  direct  current,  because  it  flows  in  the  same  di- 
rection as  the  inducing  current. 

The  reason  no  spark  is  observed  on  making  the 
circuit  of  a  long  coil,  to  which  the  terminals  of  a 
single  voltaic  cell  are  connected,  will  now  be  under- 
stood. The  differences  of  potential,  produced  on 
the  closing  of  the  circuit,  are  such  that  the  current 
thereby  generated  tends  to  flow  in  the  opposite  direc- 
tion to  that  of  the  original  current,  and  thus  de- 
creases its  strength ;  while  those  produced  on  the 
breaking  of  the  circuit  cause  a  current  that  flows  in 
the  same  direction,  and,  therefore,  prolongs  and 
strengthens  the  original  current. 

Mutual  induction  is  caused,  in  a  similar  manner, 
by  the  expanding  and  contracting  lines  of  force, 
which  are  produced  by  rapidly  varying  the  strength 
of  current  passing  in  one  circuit,  cutting  or  passing 
through  a  neighboring  circuit  and  so  producing  dif- 
ferences of  potential  therein.  These  differences  of 
potential  tend  to  produce  currents  in  one  direction 
when  the  lines  of  force  are  expanding,  and  in  the 
opposite  direction  when  they  are  contracting. 

The  effects  of  mutual  induction  can  be  readily 
shown  by  means  of  the  apparatus  represented  in 


ELECTRO-DYNAMIC  INDUCTION.  259 

Fig.  118,  where  B,  consists  of  two  concentric  coils 
of  insulated  wire  that  are  separately  wound  on  a  hol- 
low core  of  vulcanite  or  other  insulating  material. 
One  of  these  coils  is  called  the  primary  coil,  and  the 
other  the  secondary  coil.  The  terminals  of  the 
primary  coil  are  connected  to  the  poles  P  and  N,  of 
a  voltaic  cell.  The  terminals  of  the  secondary  coil 
are  connected  to  the  galvanometer  G. 

If,  now,  the  circuit  of   the  voltaic  cell  be  closed 
through  the  primary  coil,  then   at  the  moment  of 


FIG.  118.— MUTUAL  INDUCTION. 

closing  the  circuit,  a  current  is  produced  in  the 
secondary  coil,  which  flows  in  the  opposite  direction 
to  the  current  flowing  through  the  primary,  as  is  in- 
dicated by  the  deflection  of  the  needle  of  the  gal- 
vanometer in  a  certain  direction.  If  the  circuit  be 
opened,  then  at  the  moment  of  opening,  a  current  is 
produced  in  the  secondary,  which  flows  in  the  same 
direction  as  the  current  in  the  primary,  as  is  indi- 
by  the  deflection  of  the  galvanometer  needle  in  the 
opposite  direction. 


260 


ELECTRICAL  MEASUREMENTS. 


As  in  the  case  of  self  induction  these  currents 
are  but  momentary,  and  continue  only  as  long  as 
the  current  in  the  primary  varies  in  strength. 

"When  the  current  strength  is  fully  established  in 
the  primary  coil,  and  no  current  exists  in  the  second- 
ary, if  a  short  circuit  is  formed  across  the  battery 
terminals  by  placing  the  wire  d,  in  the  mercury  cups 
x  and  y,  shown  in  Fig.  119,  the  current  in  the  pri- 
mary is  thereby  decreased  and  a  current  is  induced 
in  the  secondary  coil  in  the  same  direction  as  that 
flowing  in  the  primary  circuit. 


FIG.  119.— MUTUAL  INDUCTION. 

In  magneto-electric  induction,  the  current  is  ob- 
tained by  means  of  differences  of  potential  estab- 
lished in  a  conductor,  either  by  moving  the  conduc- 
tor through  the  field  of  a  permanent  magnet,  so  as 
to  cut  its  lines  of  force,  or  by  moving  the  magnet 
past  the  conductor  so  as  to  cause  the  lines  of  force 
to  pass  across  the  conductor. 

Magneto-electric  induction  can  be  readily  shown 


ELECTRO-DYNAMIC  INDUCTION. 


261 


by  means  of  the  apparatus  represented  in  Fig.  120. 
The  terminals  of  a  coil  of  insulated  wire  are  con- 
nected to  the  terminals  of  a  galvanometer.  When 
the  magnet  M,  is  moved  toward  or  from  such  coil 
the  needle  of  the  galvanometer  will  be  deflected 
by  a  current,  which  will  flow  in  one  direction  as  the 
magnet  approaches  the  coil,  and  in  the  opposite  di- 
rection as  it  moves  away  from  it. 


Fio.  120.— MAGNETO-ELECTRIC  INDUCTION. 

In  electro-magnetic  induction,  a  coil  or  conductor, 
moved  through  the  field  of  an  electro-magnet  so  as 
to  cut  its  lines  of  force,  has  a  current  produced  in  it 
by  the  differences  of  potential  thereby  generated  in 
the  coil  or  conductor.  This  kind  of  induction  differs 
from  magneto-electric  induction  only  in  the  fact 


262  ELECTRICAL  MEASUREMENTS. 

that  the  magnet  used  is  an  electro-magnet  and  not 
a  permanent  magnet. 

Electro-magnetic  induction  can  be  shown  by 
means  of  the  apparatus  represented  in  Fig.  121.  The 
terminals  of  a  voltaic  cell  are  connected  to  the  ends 
of  a  coil  of  insulated  wire,  as  shown.  The  terminals 
of  a  second  hollow  coil,  provided  with  a  sufficiently 
wide  opening  to  permit  the  first  coil  being  moved 


Fia.  121.— ELECTRO-MAGNETIC  INDUCTION. 

into  and  out  of  it,  are  connected  to  the  terminals  of 
a  galvanometer.  When  the  coil  connected  to  the 
battery — which  may  be  called  the  primary  coil — is 
moved  into  or  out  of  the  coil  of  wire  whose  terminals 
are  connected  to  the  galvanometer — which  may  be 
called  the  secondary  coil — currents  are  produced 
in  it,  as  is  shown  by  the  deflection  of  the  needle  of 
the  galvanometer.  These  currents  flow  in  one  di- 


ELECTRO-DYNAMIC  INDUCTION.  263 

rection  as  the  primary  circuit  is  moved  toward  the 
secondary  circuit,  and  in  the  opposite  direction  as  it 
is  moved  away  from  it. 

The  following  considerations  will  show  that  the 
production  of  currents  by  electro-magnetic  induc- 
tion is  in  reality  the  same  as  their  production  by 
magneto-electric  induction. 

When  a  steady  current  *is  flowing  through  a  coil 
of  wire  placed  in  the  neighborhood  of  another  coil 
of  wire,  no  difference  of  potential  is  produced  in  the 
neighboring  wire  as  long  as  such  coils  remain  fixed. 
If,  however,  either  be  moved  toward  the  other,  so 
that  the  lines  of  force  produced  by  one  coil  are 
caused  to  pass  through  the  other  coil,  differences 
of  potential  will  be  produced,  and  the  direction  of 
the  resulting  current  will  be  opposite  to  the  current 
flowing  through  the  inducing  coil  when  they  are 
moved  toward  one  another,  and  in  the  same  direc- 
tion when  they  are  moved  from  one  another. 

In  the  same  way,  when  a  permanent  magnet  is 
moved  toward  a  conductor,  or  a  conductor  is  moved 
toward  a  permanent  magnet,  so  as  to  cause  the  lines 
of  force  to  move  across  the  conductor,  differences  of 
potential  are  produced,  which  are  respectively  in- 
verse and  direct  as  the  one  approaches  or  moves 
away  from  the  other,  and  their  directions  may  be  de- 


ELECTRICAL  MEASUREMENTS. 


duced  by  reference  to  the  direction  of  the  Amperean 
currents,  which  are  assumed  to  produce  the  magnet- 
ism of  a  permanent  magnet. 

Magneto-electric  induction  and  electro-magnetic 
induction  are,  therefore,  sometimes  called  dynamo- 
electric  induction. 


DIHECTIOK 
OF  LINE 

OF 
MOTION 


FIG.  122  —FLEMING'S  RULE. 

The  same  principles  may  be  expressed  by  the  fol- 
lowing laws  : 

(1.)  Any  increase  in  the  number  of  lines  of  mag- 
netic force  which  pass  through  a  loop  of  a  circuit, 


ELECTRO-DYNAMIC  INDUCTION. 


265 


produces  an  inverse  current  in  that  circuit ;  any  de- 
crease in  the  number  of  such  lines  produces  a  direct 
current  in  that  circuit. 

(2.)  The  intensity  of  the  induced  current,  or,  more 
correctly,  the  difference  of  potential  produced,  is 
proportional  to  the  rate  of  increase  or  decrease  of 
the  lines  of  force  passing  through  the  loop. 


Direction  of 
Motion. 


a 

o    . 


FIG.  123.— FLEMING'S  RULE. 

The  direction  of  the  currents  produced  by  dynamo- 
electric  induction  may  be  remembered  by  the  follow- 
ing plan  suggested  by  Fleming.  Let  the  right  hand 
be  held  with  the  fingers  extended  as  shown  in  Fig. 
122.  Let  the  forefinger  represent  the  positive  di- 
rection of  the  lines  of  force — that  is,  as  coming  out 
of  the  north  pole  of  the  magnet ;  then  if  a  conductor 


266  ELECTRICAL  MEASUREMENTS. 

be  moved  in  the  direction  in  which  the  thumb  points 
it  will  have  a  current  produced  in  it  by  induction, 
which  will  flow  in  the  direction  in  which  the  middle 
finger  points. 

Or,  the  same  thing  can  be  more  readily  remem- 
bered by  cutting  apiece  of  paper  in  the  shape  shown 
in  Fig.  123.  Marking  it  as  shown,  and  bending  the 
arm  upward  at  the  dotted  line,  so  as  to  form  three 
axes  at  right  angles  to  one  another,  then,  if  the  arm 
•P,  represents  the  direction  of  the  lines  of  force,  a 
conductor,  moved  in  the  direction  of  the  arm  M,  so 
as  to  cut  these  lines  at  right  angles,  has  a  current 
produced  in  it  which  will  flow  in  the  direction  of  the 
arm  C. 


ELECTRO-DYNAMIC  INDUCTION.  267 


EXTRACTS   FROM  STANDARD  WORKS. 

In  a  work  entitled  "  The  Alternate  Current  Trans- 
former in  Theory  and  Practice,"*  by  J.  A.  Fleming, 
the  following  description  &  given  on  page  36,  Vol. 
I.,  concerning  some  of  the  phenomena  of  electro- 
dynamic  induction: 

As  soon  as  we  cease  to  limit  our  consideration  to  con- 
stant or  steady  currents  we  find  that  we  shall  not  be  able 
to  give  a  full  account  of  the  phenomena  unless  we  extend 
our  ideas  and  recognize  another  quality  of  conductors 
equally  important  with  resistance  in  determining  the 
numerical  ratio  of  instantaneous  current  strength  to  in- 
stantaneous potential  difference  between  two  points  in  any 
linear  conductor  traversed  by  that  current.  This  quality 
of  the  circuit  is  called  its  Inductance. 

The  clear  recognition  of  this  special  quality  of  a  conduc- 
tor dates  from  the  publication  of  Faraday's  memoir  form- 
ing the  Ninth  Series  of  his  "  Electrical  Researches  "  (§1,048 
1st  Ed.),  On  the  Influence  by  Induction  of  an  Electric  Cur- 
rent on  Itself,  and  from  the  investigations  of  Prof.  Joseph 
Henry  (Fhil.  Mag.,  1840),  of  Princeton.  The  chain  of  ex- 
periments which  lead  to  these  ideas  was  apparently  started 
by  the  inquiry  addressed  to  Faraday  by  a  Mr.  Jen  kin  one 

*  "The  Alternate  Current  Transformer  in  Theory  and  Practice," 
by  J.  A.  Fleming,  M.A.,  D.  Sc.  2  vols.  London:  Electrician  Print- 
ing and  Publishing  Company.  1889-92.  Vol.  I.  The  Induction  of 
Electric  Currents.  487  pages,  157  illustrations.  Price,  93.  Vol.  II. 
The  Utilization  of  Induced  Currents.  594  pages,  300  illustrations. 
Price,  $5. 


268  ELECTRICAL  MEASUREMENTS. 

Friday  evening,  at  the  Royal  Institution,  as  to  the  reason 
why  a  shock  was  experienced  when  a  circuit  containing  an 
electro-magnet  was  broken,  the  observer  retaining  in  his 
two  hands  the  ends  of  the  circuit,  but  no  shock  was  felt  if 
the  circuit  contained  neither  magnet  nor  long  coils  of  wire. 
Faraday  seems  speedily  to  have  arranged  an  organized  at- 
tack on  the  subject  and  to  have  returned  from  his  investi- 
gation burdened  with  the  spoils  of  victory  in  the  shape  of 
the  following  facts  : 

(1.)  If  a  battery  circuit  is  closed  by  a  short  thick  wire, 
then,  although  there  may  be  a  strong  current  existing  in 
this  wire  on  breaking  contact  at  any  point,  little  or  no 
spark  is  seen,  and  if  the  two  ends  of  the  circuit  are  grasped 
in  the  two  hands,  and  the  interruption  takes  place  between 
the  hands,  then  little  or  no  shock  is  experienced. 

(2.)  If  a  long  wire  is  used  instead,  then,  although  the  ab- 
solute strength  of  the  current  may  be  less,  yet  the  spark 
and  shock  at  interruption  are  more  manifest. 

(3.)  If  this  length  of  insulated  wire  is  coiled  up  in  a  helix 
on  a  pasteboard  tube,  then,  although  the  length  of  the 
wire  and  the  strength  of  the  current  are  the  same,  yet  the 
spark  and  the  shock  are  still  more  marked. 

(4.)  If  the  above  helix  has  an  open  iron  core  placed  in  it, 
both  these  effects  are  yet  more  exalted, 

(5  )  If  the  same  length  of  wire  is  doubled  on  itself,  being, 
however,  insulated,  then  the  effects  nearly  vanish,  and, 
whether  straight  or  coiled,  this  double  wire  with  current 
going  up  one  side  and  down  the  other  is  no  better  in  re- 
spect of  spark  and  shock  on  interruption  than  a  very  short 
wire. 


XII L—  INDUCTION  COILS  AND  TRANS- 
FORMERS. 


An  induction  coil  consists  of  any  arrangement  of 
parts  by  means  of  which  electric  currents  may  be 
produced  by  that  variety  of  electro-dynamic  induc- 
tion called  mutual  induction. 

In  an  induction  coil  alternating  currents  passed 
through  a  conductor  arranged  in  the  form  of  a  coil, 
induce  alternating  currents  in  a  neighboring  coil  of 
wire. 

The  coil  of  wire  through  which  the  alternating 
currents  are  passed  is  called  the  primary  coil.  The 
coil  in  which  the  alternating  currents  are  produced 
is  called  the  secondary  coil. 

As  the  current  strength  in  the  primary  coil  or  con- 
ductor varies,  it  produces  a  field  whose  lines  of 
magnetic  force  expand  or  move  outward  from  the 
conductor  while  the  current  strength  increases,  and 
contract  or  move  inward  toward  the  conductor 
while  the  current  strength  decreases.  In  these 
movements  the  lines  of  force  cut  the  conductor  of 
the  secondary  coil,  either  during  their  motion 
(269) 


270  ELECTRICAL  MEASUREMENTS. 

toward  or  from  it,  and  produce  therein  differences 
of  potential,  which  cause  electric  currents  to  flow 
through  the  secondary  circuit. 

The  directions  of  the  currents  produced  in  the 
secondary  circuit  are  as  follows  : 

(1.)  Opposite  to  the  currents  in  the  primary  coil, 
on  the  making  of  the  circuit,  or  as  the  current  is  in- 
creasing, and  the  lines  of  magnetic  force  cut  the 
secondary  circuit  on  expanding. 

(2.)  In  the  same  direction  as  the  currents  in  the 
primary  coil,  on  the  breaking  of  the  circuit,  or  as 
the  current  is  decreasing,  and  the  lines  of  magnetic 
force  cut  the  secondary  circuit  on  contracting. 

The  rate  of  alternation  of  the  current  in  the  sec- 
ondary coil  is  the  same  as  that  in  the  primary. 

The  relative  values  of  the  difference  of  potential 
produced  in  the  secondary  as  compared  with  that  in 
the  primary  depend  on  the  relative  lengths  of  the 
wires  in  the  primary  and  secondary  coils.  If,  for 
example,  the  length  of  wire  in  the  secondary  coil  cir- 
cuit is  fifty  times  the  length  of  that  in  the  primary, 
the  difference  of  potential  in  the  secondary  will  be 
fifty  times  that  of  the  primary.  If,  on  the  contrary, 
the  length  of  wire  in  the  secondary  coil  is  shorter 
than  that  in  the  primary,  the  difference  of  poten- 
tial in  the  secondary  will  be  less  than  that  in  the 


INDUCTION  COILS  AND  TRANSFORMERS.        271 

primary.  If,  for  example,  the  secondary  has  a 
length  one-fiftieth  of  that  of  the  primary,  the  dif- 
ference of  potential  in  the  secondary  will  be  one- 
fiftieth  of  that  of  the  primary. 

Since  in  a  well-constructed  induction  coil  there  is 
very  little  energy  lost  in  producing  these  differences 
of  potential  by  mutual  induction,  it  will  be  readily 
understood  that  the  amount  of  energy  produced  in 
the  secondary  will  be  very  nearly  equal  to  the  amount 
expended  in  the  primary. 

Assuming  that  the  energy  expended  in  the  pri- 
mary in  the  form  of  electric  work  may  be  approxi- 
mately expressed  as  C  E,  in  which  C,  equals  the 
the  current  in  amperes,  and  E,  the  electromotive 
force  of  the  primary  in  volts ;  then,  when  there  is 
no  loss  of  energy  in  an  induction  coil.  C  E  =  C"  E'. 

From  the  above  expression  it  will  be  seen  that  in 
whatever  proportion  the  difference  of  potential  is 
increased  in  the  secondary,  by  using  more  turns  of 
wire,  in  that  proportion  must  the  strength  of  the 
secondary  current  be  decreased  ;  since  otherwise  the 
product  of  C'  and  E',  would  not  be  equal  to  the  prod- 
uct of  C  and  E.  If,  therefore,  the  difference  of 
potential  is  increased  in  the  secondary  the  current 
strength  in  the  secondary  must  be  proportionally 
decreased. 


272  ELECTRICAL  MEASUREMENTS. 

If,  on  the  contrary,  the  difference  of  potential  in 
the  secondary  be  decreased  by  using  fewer  turns  of 
wire,  so  far  then  must  the  current  strength  of  the 
secondary  be  increased. 

There  thus  arise  two  distinct  varieties  of  induc- 
tion coils,  namely  : 

(1.)  Those  in  which  the  wire  in  the  secondary 
coil  is  longer  than  that  in  the  primary,  and  in 
which  the  difference  of  potential  induced  in  the  sec- 
ondary is,  therefore,  greater  than  that  in  the  primary, 
and  the  current  strength  in  the  secondary  is  less 
than  that  in  the  primary. 

(2.)  Those  in  which  the  length  of  wire  in  the  sec- 
ondary coil  is  shorter  than  that  in  the  primary  coil, 
and  in  which  the  difference  of  potential  induced  in 
the  secondary  is  therefore  smaller  than  that  in  the 
primary,  and  the  current  strength  in  the  secondary 
is  greater  than  that  in  the  primary. 

Originally,  induction  coils  were  made  of  the  first 
form  only ;  when,  therefore,  they  came  to  be  made 
of  the  second  form — namely,  in  which  the  length  of 
the  conductor  of  the  secondary  is  shorter  than  that 
of  the  primary — they  were  called  inverted  induction 
coils. 

A  well-known  form  of  induction  coil,  named  after 
its  inventor  the  Ruhmkorff  coil,  belongs  to  the  first 


INDUCTION  COILS  AND  TRANSFORMERS.       273 

class,  or  that  in  which  the  secondary  wire  is  of  much 
greater  length  than  the  primary. 

The  general  construction  of  this  form  of  coil  is 
shown  in  Fig.  124.  The  primary  conductor  consists 
of  but  a  few  turns  of  thick  wire,  wound  on  a  core 
formed  of  a  bundle  of  soft  iron  wires,  and  having  its 
ends  brought  out  at/,/. 


FIG.  124  — RUHMKOKFF  COIL. 

The  resistance  of  the  primary  coil  is  made  low  in 
order  that  a  strong  current  may  be  passed  through 
it. 

The  long,  thin  wire  forming  the  secondary  is 
wrapped  on  the  surface  of  a  cylinder  of  hard  rubber 
or  glass  surrounding  the  primary  coil.  In  some  coils 
this  wire  is  more  than  one  hundred  miles  in  length. 

The  primary  coil  will  induce  more  powerful  effects 
in  the  secondary  if  it  is  provided  with  a  core  of  iron. 
In  order  to  prevent  much  of  the  energy  of  the  pri- 


274  ELECTRICAL  MEASUREMENTS. 

mary  from  being  wasted  in  producing  induction  cur- 
rents in  this  core  it  is  made  of  soft  iron  wires. 

When  the  primary  circuit  is  rapidly  made  and 
broken,  alternating  currents  are  produced  in  the 
secondary  of  great  electromotive  force  but  of  small 
current  strength. 

The  making  and  breaking  of  the  primary  circuit, 
in  the  form  of  Ruhmkorff  coil  shown  in  the  figure, 
is  effected  by  means  of  a  conductor  dipping  into  a 
mercury  cup,  as  shown  at  M, 

The  more  suddenly  the  primary  circuit  is  made 
and  broken  the  greater  is  the  effect  produced  in  the 
secondary.  When  a  strong  current  is  passing 
through  the  primary  a  spark  is  formed  between  the 
contact  points,  at  which  the  primary  circuit  is  made 
and  broken.  This  spark  prolongs  the  duration  of 
the  primary  current  and  thus  decreases  its  efficiency. 
In  order  to  prevent  the  formation  of  this  spark  a 
condenser  is  generally  connected  to  the  primary  cir- 
cuit in  the  manner  shown  in  Fig.  125,  which  repre- 
sents diagramatically  the  arrangement  of  the  differ- 
ent parts  of  an  induction  coil.  The  core  consists  of 
a  bundle  of  soft  iron  wires  as  shown  at .  /,  /'. 
The  primary  wire  P  P,  consists  of  a  few  turns  of 
thick  wire,  while  the  secondary  S  S,  consists  of 
many  turns  of  fine  wire.  In  order,  however,  to 


INDUCTION  COILS  AND  TRANSFORMERS.       275 

prevent  confusion  of  details,  the  secondary  is  repre- 
sented in  the  figure  as  consisting  of  but  a  few  turns 
of  wire. 

The  terminals  of  the  battery  B,  are  connected  to 
the  primary  circuit  in  the  manner  shown.  The 
automatic  contact-breaker — provided  for  obtaining 
the  rapid  alternations  of  the  primary  circuit — is 
shown  at  H  and  0.  The  piece  of  soft  iron  H,  is 
S/ 


PIG.  125.— CIRCUIT  CONNECTIONS  OF  INDUCTION  Con., 
supported  on  the  metal  spring  h,  near  the  soft  iron 
core  /'.  When  no  current  is  passing  through  the 
primary  circuit — that  is,  when  it  is  open  or  broken — 
the  spring  rests  against  the  platinum  contact  point 
0;  but  when  the  circuit  is  closed,  the  battery  cur- 
rent flows  through  the  primary  coil,  and  magnetizes 
the  core  /,  /',  which  then  attracts  the  iron  piece  //, 


276  ELECTRICAL  MEASUREMENTS, 

thus  breaking  the  circuit  of  the  battery.  But  this 
in  turn  causes  the  core  I,  to  regain  its  magnetism' 
and  therefore  the  piece  H,  again  comes  to  rest  with 
the  spring  resting  against  the  contact  point  0,  and 
in  this  way  a  rapid  to-and-f  ro  motion  of  the  mass  H, 
is  obtained,  or  a  rapid  making-and-breaking  of  the 
battery  circuit. 

The  principles  involved  in  the  production  of  cur- 
rents by  mutual  induction  were  discovered  by  Prof. 
Henry,  who  was  the  first  to  point  out  the  true  cause 
of  the  extra  spark  produced  on  breaking  the  circuit 


a 
FIG.  126.— HENRY'S  INDUCTION  COILS. 

of  a  comparatively  long  coil  of  insulated  wire.  Henry 
also  showed  that  the  current  from  the  secondary  of 
one  induction  coil  could  be  passed  through  the  pri- 
mary of  another,  and  so  on,  thus  intensifying  the 
effects. 

A  series  of  three  of  Henry's  induction  coils  is 
shown  in  Fig.  126.  An  alternating  current  sent 
through  a,  will  produce  induced  currents  in  its 
secondary  5  ;  now,  ~b,  is  connected  in  series  with  an- 
other primary  coil  c,  whose  secondary  d,  is  similarly 


INDUCTION  COILS  AND  TRANSFORMERS.        277 

connected  to  another  primary  e.  The  currents  in- 
duced in  its  secondary  /,  are  finally  employed  to 
magnetize  a  piece  of  iron  wire  g,  or  are  used  for  any 
other  desired  effect. 

Henry  called  the  currents  produced  at  b,  secondary 
currents,  or  currents  of  the  second  order ;  those  pro- 
duced at  d,  tertiary  currents,  or  currents  of  the 
third  order ;  those  produced  at  /,  he  called  currents 
of  the  fourth  order,  and  so  on. 

In  Fig.  127  are  represented  two  such  coils  ar- 
ranged for  giving  a  person  an  electric  shock  on 
grasping  the  handles  at  e  and/. 


in 
FIG.  127.-HENRY's  INDUCTION  COILS. 

Since  an  induction  coil  converts  or  transforms  one 
form  of  electric  energy  into  another  it  is  often  called 
a  converter  or  transformer.  The  latter  name  is  the 
one  most  frequently  employed.  Although  the  term 
transformer  is  more  generally  used  in  connection 
with  the  inverted  induction  coil,  yet  it  is  now  often 
applied  to  induction  coils  like  the  Ruhmkorff,  in 
which  the  length  of  the  secondary  greatly  exceeds 
that  of  the  primary. 


278 


ELECTRICAL  MEASUREMENTS. 


Ill  order  to  distinguish  the  two  forms  of  induction 
coils  or  transformers  the  one  which  increases  the 
electromotive  force  is  called  a  step-up  transformer, 
and  the  one  which  decreases  the  electromotive  force 
is  called  a  step-down  transformer. 


p 


Fia.  128.-  CLOSED  CIRCTTITKD  TRANSFORMER. 

The  transformer  shown  in  Fig.  128  is  a  step-down 
transformer.  It  consists  essentially  of  an  inverted 
induction  coil  in  which  the  primary  P,  P,  is  formed 
of  many  turns  of  a  wire  that  is  thin  when  compared 
to  the  secondary  S,  S,  which  is  formed  of  a  few 
turns  of  comparatively  thick  wire.  In  order  to  pre- 


INDUCTION  COILS  AND  TRANSFORMERS.        279 

vent  loss  of  energy  by  the  production  of  currents  in 
the  core,  this  part  of  the  transformer  is  thoroughly 
laminated.  In  order  to  lower  the  resistance  of  the 
magnetic  circuit,  the  transformer  shown  in  the 
figure  is  iron-clad. 

Step-down  transformers  are  employed  in  systems 
of  electric  light  distribution  where  currents  of  com- 
paratively small  current  strength  and  considerable 
difference  of  potential  are  sent  over  a  line  from  a 
distant  station  into  transformers,  placed  where  the 
electric  energy  is  to  be  used,  by  which  they  are 
changed  into  currents  of  comparatively  small  differ- 
ence of  potential  and  considerable  current  strength. 

Transformers  may  be  divided,  according  to  the 
form  of  their  core,  into  closed  circuited  and  open- 
circuited  transformers.  In  the  former  the  iron  com- 
pletely surrounds  the  coils,  in  the  latter  it  only  par- 
tially surrounds  them.  The  iron-clad  transformer 
above  described  belongs  to  the  closed-circuited  class. 
That  shown  in  Fig.  129,  belongs  to  the  open-circuited 
class.  The  energy  produced  in  the  secondary  is 
somewhat  smaller  than  that  expended  in  the  primary 
on  account  of  the  following  losses: 

(1.)  Specific  inductive  capacity. 

(2.)  Hysteresis,  or  magnetic  friction. 

(3.)  Heating  of  the  primary  circuit. 


280  ELECTRICAL  MEASUREMENTS. 

(4.)  Heating  of  the  secondary  circuit. 

(5.)  Foucault  currents. 

When  a  converter  is  properly  constructed  the  loss 
of  conversion  at  full  load  is  but  small,  the  number 
of  watts  in  the  secondary  being  very  nearly  equal  to 
those  in  the  primary.  A  current  of  ten  amperes  at 
2,000  volts,  when  passed  into  a  converter,  the  num- 
ber of  whose  primary  turns  is  twenty  times  the 
number  of  its  secondary  turns,  will  produce  a  current 


FIG.  129.— OPEN-CIRCUITED  TRANSFORMER. 

in  its  secondary  whose  strength  is  about  twenty 
times  as  great,  or  nearly  200  amperes,  but  whose 
voltage  is  about  one-twentieth,  or  100  ;  the  watts  in 
the  two  cases  are  nearly  the  same,  or,  theoretically, 
20,000  watts.  In  reality  it  is  somewhat  smaller. 

In  a  form  of  apparatus  known  as  the  pyromag- 
netic  generator,  electric  currents  are  obtained  by  a 
species  of  electro-dynamic  induction.  Here,  how- 


INDUCTION  COILS  AND  TRANSFORMERS.        281 

ever,  the  energy  of  heat  is  employed  to  produce  al- 
ternations in  the  strength  of  the  magnetism.     As  is 


FIG.  130.— PYROMAGNETIC  GENERATOR. 
well  known,  the  strength  of   magnetism  in  iron  de- 
creases with  an  increase  of  temperature.     If,  then, 


282  ELECTRICAL  MEASUREMENTS. 

means  are  provided  by  which  a  magnetic  mass  of 
iron  is  alternately  heated  and  cooled  the  varying 
strength  of  its  magnetism  will  produce  expanding 
and  contracting  lines  of  magnetic  force,  which  may 
be  caused  to  cut  a  neighboring  coiled  conductor  and 
thus  produce  differences  of  potential  therein. 

A  pyromagnetic  generator  is  shown  in  Fig.  130. 
This  apparatus  is  sometimes  called  a  pyromagnetic 
dynamo.  Eight  electro-mug  nets,  placed  as  shown, 
are  provided  each  with  an  armature  consisting  of  a 
roll  of  corrugated  iron  that  has  a  coil  of  insulated 
wire  wound  on  it,  protected  by  asbestos  paper.  These 
armatures  pass  through  two  iron  discs,  as  shown,  and 
have  their  coils  connected  in  series  in  a  closed  cir- 
cuit, the  wires  from  the  coils  being  connected  with 
metallic  brushes  that  rest  on  a  commutator  supported 
on  a  vertical  axis.  A  pair  of  metallic  rings  is  pro- 
vided to  carry  off  the  current  generated. 

The  vertical  axis  is  provided  below  with  a  semi- 
circular screen  called  a  guard-plate,  that  rotates 
with  the  axis  and  cuts  off  or  screens  one-half  of  the 
armature  from  heated  air,  generated  below  in  a  fire. 
The  difference  in  the  magnetization  of  the  arma- 
tures, when  hot  and  cold,  produces  expanding  and 
contracting  lines  of  force,  which  produce  electric 
currents. 


INDUCTION  COILS  AND  TRANSFORMERS, 


EXTRACTS  FROM  STANDARD   WORKS.' 

J.  A.  Fleming  in  his  "  Short  Lectures  to  Elec- 
trical Artisans/'  *  on  page  75,  says  as  regards  induc- 
tion coils  : 

Very  great  precautions  as  to  insulation  are  essential,  in 
order  to  obtain  long  sparks  from  induction  coils.  In  a 
large  coil,  built  for  the  late  Mr.  Spottiswoode,  the  secondary 
coil  had  a  total  length  of  about  280  miles,  and  the  primary 
a  total  length  of  1,164  yards. 

Mr.  Spottiswoode  obtained  very  powerful  discharges  from 
his  coil  by  disconnecting  the  interrupter  and  condenser  and 
sending  direct  into  the  primary  the  alternate  currents  of  a 
De  Meritens  alternate  current  magneto-machine. 

Induction  coils,  such  as  above,  formerly  found  their  ap- 
plications only  in  scientific  research  and  experiments,  but 
they  have  recently,  under  modifications,  become  important 
practical  appliances  in  electric  lighting,  and  in  this  appli- 
cation are  called  secondary  generators. 

An  induction  coil  is  a  reversible  machine.  If  a  current 
of  considerable  magnitude  circulates  under  small  electro- 
motive force  in  the  primary,  then  variations  in  the  strength 
of  this  give  rise  to  very  small  currents  of  exceeding  high 
electromotive  force  in  the  secondary.  We  may  reverse  this 

*  "  Short  Lectures  to  Electrical  Artisans  :  Bemg  a  Course  of  Ex- 
perimental Lectures  Delivered  fco  a  Practical  Audience,"  by  J.  A. 
Fleming,  M.A.,  D.fc.  London  :  E.  &  F.  N.  Spon.  1892.  210  pages, 
74  illustrations.  Price,  f  1.50. 


384  ELECTRICAL  MEASUREMENTS. 

induction,  and  cause  to  circulate  in  the  secondary  very 
small  currents  under  very  high  electromotive  force.  These 
by  their  fluctuations,  will  generate  in  the  primary  large 
currents  of  small  electromotive  force.  We  do  not,  in  either 
case,  create  electric  energy.  The  energy  of  a  current  flow- 
ing in  a  conductor  at  any  instant  is  measured  by  the  product 
of  the  current  strength  and  the  electromotive  force  between 
the  ends  of  that  conductor,  and  hence  electric  energy  is  a 
quantity  which  is  the  product  of  two  factors,  current  and 
electromotive  fo~ce.  What  the  induction  coil  enables  us  to 
do  is  to  increase  one  of  these  factors  at  the  expense  of  the 
other,  and  transform  our  electric  energy  in  form,  but  not 
in  amount.  In  this  respect  we  operate  on  electric  energy 
by  means  of  an  induction  coil,  just  as  a  simple  mechanical 
power,  such  as  a  pulley,  enables  us  to  operate  on  mechanical 
energy,  converting  a  quantity  of  work  which  consists  of  a 
small  stress  exerted  through  a  great  distance  into  a  large 
stress  exerted  through  a  small  distance. 

Fleming,  in  the  second  volume  of  the  "Alternate 
Current  Transformer,"*  speaking  of  the  historical 
development  of  the  induction  coil  and  transformer, 
says  on  page  1: 

In  following  out  the  stages  of  development  of  the  induc- 
tion coil  and  the  transformer,  we  find  that  they  are  no  ex- 
ceptions to  the  general  law  that  improvements  in  experi- 


*  "  The  Alternate  Current  Transformer  in  Theory  and  Practice, " 
by  J.  A.  Fleming.  Vol.  II.,  "  The  Utilization  of  Induced  Cur- 
rents." London :  The  Electrician  Printing  and  Publishing  Com- 
pany. 1892.  594  pages,  300  illustrations.  Price,  $5. 


INDUCTION  COILS  AND  TRANSFORMERS.       285 

mental  appliances  advance  along  definite  lines  by  a  process 
of  evolution  in  which  rudimentary  forms  are  successively 
replaced  by  more  and  more  completely  developed  machines. 
We  are  able,  by  a  careful  scrutiny  of  existing  and  pre- 
existing modifications,  to  detect  the  ideas  which  at  every 
step  have  impelled  inventions  forward,  and  also  to  examine 
the  prototypes  in  their  relation  to  the  final  and  fully  de- 
veloped idea.  Most  readers  would  probably  consider  that  the 
prototypical  form  of  all  modern  induction  coils  and  trans- 
formers was  the  iron  ring  with  which  Faraday  made  the 
initial  discovery  in  electro-magnetic  induction,  and  in  one 
sense  this  is  of  course  correct ;  but  a  careful  examination  of 
the  early  stages  of  the  induction  coil  as  we  now  possess  it 
seems  to  show  that  it  is  descended  in  a  direct  line  not  from 
Faraday's  ring  so  much  as  from  Henry's  flat  spirals,  and 
that  it  is  these  latter  which  are  the  chiefs  of  the  clan  and 
the  true  ancestors  of  our  modern  coil. 

Henry's  claim  to  be  an  independent  discoverer  of  the 
fundamental  fact  of  electro-magnetic  induction  is  not  now 
disputed.  In  the  July  number  of  Silliman'ls  American 
Journal  of  Science  for  1832,  Joseph  Henry,  then  a  young 
teacher  in  the  Albany  Academy,  gave  an  account  of  the 
manner  in  which  he  had  independently,  and  before  receiv- 
ing an  account  of  Faraday's  work,  performed  in  the  previ- 
ous autumn,  elicited  from  his  own  great  electro-magnet  an 
induced  current  by  wrapping  round  the  soft  iron  arma- 
ture certain  coils  of  insulated  wire.  In  the  same  paper  in 
which  he  thus  discloses  his  anticipated  discovery  he  ren- 
dered an  account  of  that  in  which  he  had  in  turn  antici- 
pated his  illustrious  rival  by  the  discovery  of  the  fact  of  the 


286  ELECTRICAL  MEASUREMENTS. 

self-induction  of  a  spiral  conductor,  and  denoted  the  phe- 
nomena by  which  it  has  since  been  known.  Simply  confin- 
ing himself  to  the  bare  statement  of  the  new  fact  that  if  the 
poles  of  a  small  battery  are  joined  by  a  wire  a  foot  long  no 
spark  will  be  found  on  breaking  the  circuit,  whereas  if 
the  wire  be  thirty  or  forty  feet  long,  and  particularly  if  it 
be  coiled  into  a  spiral,  it  gives  a  bright  spark  when  so  em- 
ployed, Henry  noted  the  discovery  and  correctly  attrib- 
uted the  phenomena  to  the  induction  of  the  circuit  upon 
itself.  Finding,  however,  that  Faraday  was  following  on 
the  same  line  of  discovery  he  published  in  the  Journal  of 
the  Franklin  Institute,  in  March,  1835  (Vol.  XV.,  pp.  169, 
170),  a  brief  epitome  of  the  facts  he  had  collected,  and 
made  mention,  for  the  first  time,  of  the  use  of  the  spirals 
of  flat  copper  tape  or  ribbon,  insulated  and  closely  wound 
together,  with  which  he  subsequently  conducted  his  brill- 
iant train  of  discoveries  on  the  mutual  induction  of  cir- 
cuits. 


XIV.— DYNAMO-ELECTRIC  MACHINES. 


A  dynamo-electric  machine  is  any  combination 
of  parts  by  which  mechanical  energy  is  converted 
into  electrical  energy  by  means  of  electro-dynamic 
induction,  that  is,  by  causing  conductors  to  pass 
through  or  to  cut  lines  of  magnetic  force. 

The  term  "  dynamo-electric  machine  "  is  some- 
times applied  not  only  to  machines  in  Which  mechan- 
ical energy  is  converted  into  electrical  energy,  but 
also  to  those  in  which  electrical  energy  is  converted 
by  motors  into  mechanical  energy.  The  term  "elec- 
tric motor/'  however,  is  preferable  for  the  latter  case, 
and  is  now  generally  so  employed.  Sylvanus  P. 
Thompson  in  his  "Dynamo-Electric  Machinery " 
defines  a  dynamo-electric  machine  as  follows  : 

' '  A  machine  for  converting  energy  in  the  form  of 
mechanical  power  into  energy  in  the  form  of  electric 
currents,  or  vice  versa,  by  the  operation  of  setting 
conductors  (usually  in  the  form  of  coils  of  copper 
wire)  to  rotate  in  a  magnetic  field,  or  by  varying  a 
magnetic  field  in  the  presence  of  conductors. " 

Originally  the  term  "dynamo-electric  machine"  was 

(287) 


288  ELECTRICAL  MEASUREMENTS. 

limited  to  the  case  of  a  machine  for  converting 
mechanical  power  into  electrical  power,  in  which  the 
machine  was  self  exciting,  that  is,  required  no  other 
current  to  start  it  than  that  produced  when  its  arm- 
ature was  rotated  in  the  permanent  field  of  the  ma- 
chine. 

Dynamo-electric  machines  are  now  constructed  in 
a  great  variety  of  forms,  consisting,  however,  of  the 
following  parts  : 

(1.)  A  rotating  part  called  the  armature,  which 
consists  of  coils  of  wire,  or  conducting  bars,  strips, 
or  plates,  generally  placed  on  a  core  of  soft  iron 
and  rotated  in  the  magnetic  field  of  the  machine  so 
as  to  cut  its  lines  of  magnetic  force.  Sometimes  the 
armature  is  stationary  and  the  field  rotates  or  pul- 
sates. 

(2.)  The  field  magnets  which  produce  the  mag- 
netic field  in  which  the  armature  rotates. 

(3.)  The  pole  pieces  of  the  field  magnets,  which 
are  designed  to  distribute  the  field  of  the  field  mag- 
nets evenly  over  the  rotating  armature  and  to  reduce 
the  resistance  of  the  air  gap. 

(4.)  The  commutator  by  means  of  which  the 
currents,  produced  in  the  armature  by  the  differences 
of  potential  generated  in  its  conductors  on  rotation, 
are  caused  to  flow  in  one  and  the  same  direction. 


DYNAMO-ELECTRIC  MACHINES.  289 

In  alternating  cm-rent  dynarno^electric  machines, 
in  which  the  currents  produced  by  the  armature  are 
not  caused  to  flow  in  one  and  the  same  direction, 
but  flow  in  alternately  opposite,  directions,  this  part 
of  the  dynamo-electric  machine  is  called  the  col- 
lector, since  it  merely  serves  to  collect  the  currents. 


FIG.  131.— SERIES  DY&A-MO. 

(5.)  The  collecting  brushes  .that  rest  on  the,  .091x1- 
mutator  cylinder  an4  carry  off  .the  current  produced 
in  the  coils  by  the  differences  of  potential  generated 
therein  on  their  rotation  through  the  field.  ,-_ir 

The  relations  which  these  different  parts  bear  to 
one  another  can  be -seen-  in  Fig.  131;  in  which  is 
shown,  a  form  of  •  dynamo-electric  machine.  -  TJ^e 


290  ELECTRICAL  MEASUREMENTS. 

field  magnets  consist  of  four  coils  of  wire  so  placed 
on  a  heavy  core  of  soft  iron,  called  the  field  magnet 
frame,  as  to  produce  consequent  poles,  that  is, 
two  north  poles,  for  example,  in  a  massive  piece  of 
iron  called  a  pole  piece,  attached  to  the  frame  of 
the  machine  and  placed  at  the  top  of  the  machine, 
and  two  south  poles  in  a  similar  pole  piece  placed  at 
the  bottom  of  the  machine.  These  pole  pieces  are 


FIG.  132.— DKUM-ARMATURB. 

placed  and  shaped  so  as  to  conform  to  the  cylin- 
drical outline  of  the  armature  which  rotates  between 
them. 

The  armature  consists,  as  shown,  of  many  coils  of 
wire  wound  around  a  ring  or  hollow  cylinder,  the 
ends  of  contiguous  coils  being  connected  together 
and  to  insulated  segments  of  the  commutator  on 


DYNAMO-ELECTRIC  MACHINES.  291 

which  the  collecting  brushes  rest  that  carry  off  the 
current. 

A  great  variety  of  shapes  may  be  given  to  the  ar- 
matures of  dynamo?. 

A  very  common  form  of  armature,  called  the  drum- 
armature,  shown  in  Fig.  132,  takes  its  name  from 
the  drum  shape  of  the  core  on  which  the  wire  is 
wound.  As  will  be  seen  from  an  inspection  of  the 


Fia.  133.— RING-ARMATURE. 

figure,  the  armature  coils  are  wound  longitudinally 
over  the  surface  of  a  closed  drum  or  cylinder,  and 
the  ends  are  afterward  connected,  in  the  manner 
shown,  to  insulated  plates  of  metal,  suitably  sup- 
ported and  arranged  in  the  form  of  a  cylinder  called 
the  commutator  cylinder.  In  this  case  the  collect- 
ing brushes  B,  B,'  rest  on  the  commutator  cylinder 


292  ELECTRICAL  MEASUREMENTS. 

in  the  positions  shown,  and  carry  off  the  current 
from  the  armature. 

Another  form  of  armature  called  the  ring-arma- 
ture, from  the  ring-shape  of  its  core,  is  shown  in 
Fig.  133.  In  a  ring-arniature  the  coils  are  con- 
nected to  one  another  and  to  the  separate  pieces  of 
metal  in  the  commutator  cylinder,  as  shown  in  the  fig- 
ure ;  namely,  by  connecting  the  beginning  and  end 
of  contiguous  coils  together,  and  to  a  separate  seg- 
ment of  the  commutator  cylinder.  It  will  be  no- 


Fio.  134.— POLE-ARMATURE. 

ticed  that  the  number  of  separate  parts  or  segments 
in  a  commutator  cylinder  will  depend  either  on 
the  number  of  separate  coils  that  are  wound  on  the 
armature  core,  or  on  the  number  of  separate  pairs  of 
coils  that  are  first  connected  together,  and  after- 
ward connected  at  their  common  junction  to  the 
commutator  segment. 

The  form  of  armature  shown  in  Fig.  134  is 
called  a  pole-armature.  It  consists  of  a  series  of 
coils  of  insulated  wire  wound  on  cylindrical  cores 


DYNAMO-ELECTRIC  MACHINES.  293 

that  project  radially  from   the  periphery  of  a  disc, 
drum  or  ring. 

As  the  armature  is  rapidly  rotated  in  the  magnet- 
ic field  produced  by  the  two  field  magnets  N  and  S, 
it  cuts  their  lines  of  magnetic  force,  and  has  differ- 
ences of  potential  generated  in  its  wires  or  conduct- 
ors, that,  when  such  are  connected  to  closed  cir- 
cuits, produce  currents  which  flow  in  one  direction 
during  motion  past  one  of  the  magnet  poles,  and  in 


FIG.  135.— COMMUTATOR  OF  DYNAMO-ELECTRIC  MACHINE. 

the  opposite  direction  during  motion  past  the  other 
magnet  pole.  In  order  to  cause  such  currents  to 
flow  in  one  and  the  same  direction  they  are  corn- 
mutated  or  changed  in  their  direction  by  the  action 
of  the  commutator. 

The  operation  of  the 'commutator  will  be  under- 

-  stood  from  an  inspection  of  Fig.  135,  which  shows 

the  action  that  occurs  in  the  case  of  a  coil  of  wire 

rotated  between  two  magnet  poles.     One  end  of  such 


294  ELECTRICAL  MEASUREMENTS. 

a  coil  is  connected  to  the  insulated  segment  A',  and 
the  other  end  to  the  insulated  segment  A. 

The  brushes  B  and  B',  are  so  placed  on  the  com- 
mutator cylinder  that  they  are  in  contact  with  the 
segments  A'  and  A,  respectively,  as  long  as  the 
current  flows  in  the  same  direction  in  the  coil  of 
wire,  but  are  in  contact  with  A  and  A',  when  the 
current  changes  its  direction,  and  continue  in  such 
contact  as  long  as  the  current  flows  in  this  direction. 
By  these  means,  therefore,  the  current  will  flow  in 
one  and  the  same  direction  through  the  circuit  con- 
nected with  the  collecting  brushes. 

Since  the  commutator  segments  are  subject  to 
wear,  both  from  friction  of  the  brushes  and  the 
burning  action  of  destructive  sparks,  they  are  gen- 
erally made  of  comparatively  thick  metal;  they  are 
insulated  from  one  another  and  suitably  supported, 
generally  by  a  rocker  arm,  on  the  shaft  of  the 
armature.  The  number  of  metal  segments  placed 
on  the  commutator  cylinder  -will  depend  on  the 
number,  arrangement  and  connection  of  the  arma- 
ture coils;  it  is  generally  equal  to  the  number  of  com  • 
plete  coils. 

In  Fig.  136,  a  single  coil  A  B,  is  shown  with  its  ends 
connected  to  the  two-part  commutator^  C,'  and  placed 
so  as  to  be  capable  of  rotation  around  the  axis  R  R. 


DYNAMO  ELECTRIC  MACHINES.  295 

The  same  connections  will  be  made  whether  the 
coil  is  formed  of  a  single  turn  or  of  two  or  more 
turns  of  wire.  For  example,  in  Fig.  137  is  shown 


Rf 
FIG.  136.— CONNECTION  OF  COIL  TO  COMMUTATOR  SEGMENTS. 

a  coil  A  B,  consisting  of  two  turns  similarly 
connected  to  the  segments  of  a  two-part  commu- 
tator. So  also  in  Fig.  138  a  similar  connection  is 
shown  of  a  coil  placed  on  the  ring-armature  G. 


FIG.  137.— TWO-PART  COMMUTATOR. 

The  field  magnets  of  a  dynamo-electric  machine 
consist  of  a  frame  or  core  on  which  the  magnetizing 
coils  are  wound. 


296  ELECTRICAL  MEASUREMENTS. 

The  field  magnet  cores  should  be  made  of  thick 
solid  iron  as  soft  as  possible,  the  great  size  being 
necessary  in  order  to  ensure  a  powerful  magnetic 
field  so  as  to  ensure  a  high  voltage  as  well  as  to 
prevent  the  magnetizing  effect  of  the  armature  from 
too  greatly  influencing  the  field  of  the  field  magnets. 

The  pole  pieces  should  also  be  massive  and  made 
of  very  soft  iron.  They  may,  if  so  desired,  be  lamin- 
ated so  as  to  avoid  a  loss  of  energy  from  the  produc- 

A 


FIG.  138.— CONNECTION  OF  COIL  TO  COMMUTATOR  SEGMENTS. 

tion  therein  of  currents  called  eddy  currents.  The 
pole  pieces  should  preferably  extend  partly  around 
the  armature  so  as  to  cause  the  lines  of  force  of  the 
field  magnets  to  be  distributed  as  "much  as  possible 
over  the  armature  surface,  and  to  reduce  the  re- 
sistance of  the  air  gap.  Care  must  be  taken, 
however,  in  bringing  the  edges  of  the  opposite  pole 
pieces  near  together,  since  otherwise  the  lines  of ' 


DYNAMO-ELECTRIC  MACHINES.  297 

force  might  pass  directly  through  the  air  between 
the  edges  of  the  pole  pieces,  rather  than  through  the 
armature  itself. 

The  collecting  brushes  consist  of  strips  of  metals, 
bundles  of  wire,  slotted  plates  of  metal,  or  plates  of 
carbon  that  bear  on  the  commutator  cylinder  and 
carry  off  the  current. 

Various  forms  are  given  to  the  brushes.  The 
commonest,  however,  are  shown  in  Fig.  139,  where 


FIG.  139.—  BRUSHES. 


the  brush  B,  is  formed  of  copper  Avire  soldered  to-  , 
gether  at  its  non-bearing  end  B;  that  at  C,  is  formed  of 
a  plate  of  copper  split,  as  shown,  at  its  non-bearing 
end.  Brushes  of  carbon  are  very  commonly  employed- 
Let  us  now  consider  a  single  loop  or  coil  of  wire, 
such,  for  example,  as  that  shown  at  A  B  C  D,  in  Fig. 
140,  supported  so  as  to  rotate  between  the  poles  6' 
N,  of  the  field  magnets. 


298  ELECTRICAL  MEASUREMENTS. 

The  ends  of  the  coil  are  connected  in  the  manner 
shown  to  the  two-part  commutator.  On  the  rota- 
tion of  the  coil  so  that  the  top  of  the  coil 
shall  move  in  the  direction  of  the  large  arrow, 
differences  of  potential  are  generated  which  will 
cause  an  electric  current  to  flow  in  the  direction 
shown  by  the  smaller  arrows  during  the  motion  of 
the  loop  past  the  north  pole  from  the  bottom  to  the 
top.  In  other  words,  currents  are  produced  which 
flow  in  one  direction  during  one-half  of  its  rotation, 

A 


FIG.  UO.— INDUCTION  IN  ARMATURE  LOOP. 

and  in  the  opposite  direction   during  the  other  half 
of  its  rotation. 

If,  now,  collecting  brushes  rest  on  the  commuta- 
tor cylinder  in  the  position  shown  in  Fig.  141,  the 
current  will  flow  in  one  and  the  same  direc- 
tion, and  B',  will  become  the  positive  brush  because 
it  will  be  connected  with  the  end  of  the  coil  only  while 
it  is  sending  its  current  into  such  brush,  and  since,  as 
soon  as  the  direction  of  the  current  in  the  coil  is 


DYNAMO-ELECTRIC  MACHINES.  299 

changed,  the  other  end  will  by  the  rotation  be  moved 
into  connection  with  the  said  brush,  so  that  the  cur- 
rent generated  in  the  armature  will  be  constantly 
passed  into  the  brush  E >',  which  will  thus  become 
the  positive  brush. 

Theoretically  the  points  where  the  brushes  rest  on 
the  commutator  cylinder  will  fall  in  the  vertical  line 
coinciding  with  the  space  between  the  poles.  That  is 
to  say,  the  diameter  of  commutation,  or  the  line  con- 
necting the  points  on  the  commutator  cylinder  where 


FIG.  111.— ACTION  OF  COMMUTATOR. 

the  brushes  rest, will  take  this  direction.  In  practice, 
however, this  line  is  frequently  shifted  in  the  direction 
of  rotation  on  account  of  the  reaction  which  occurs 
between  the  magnetic  poles  of  the  field  and  the 
magnetic  poles  of 'the  armature,  as  shown  in  Fig.  142. 

Dynamo-electric  machines  may  be  divided  into  dif- 
ferent classes. 

(1.)  According  to  the  number  and  disposition  of 
the  magnetic  fields  into  unipolar,  bipolar  and  multi- 
polar  machines. 


300  ELECTRICAL  MEASUREMENTS. 

(2.)  According  to  the  manner  in  which  the  mag- 
netization of  the  field  magnets  is  obtained,  into  self- 
excited  and  separately-excited  machines. 

(3.)  According  to  the  character  of  the  connec- 
tions between  the  circuit  of  the  magnetizing  coils, 
the  armature  circuit,  and  the  circuit  external  to  the 
machine. 

(4.)  According  to  the  character  of  the  separate 
coils,  on  the  field  magnets,  into  simple  and  com- 
pound-wound machines. 


FIG.  142.— CAUSE  OP  LEAD  OP  BRUSHES. 

(5.)  According  to  the  character  of  the  armature 
winding,  or  the  shape  of  the  armature  itself. 

(6.)  According  to  whether  the  current  developed 
in  the  armature  is  rendered  continuous  or  is  left  al- 
ternating. 

We  will  here  discuss  only  those  varieties  of 
machines  that  arise  from  the  different  ways  in 
which  the  circuit  of  the  field  magnet  coils,  the  arma- 


DYNAMO-ELECTRIC  MACHINES.  301 

ture,  and  the  external  circuit  are  connected  to  one 
another.  The  most  important  of  such  varieties 
are  : 

(1.)  The  Series  Dynamo. 

(2.)  The  Shunt  Dynamo. 

(3.)  The  Separately-Excited  Dynamo. 


D    D    D   D 
FIG.  143.-SERIE3  DYNAMO. 

(4.)  The  Series-and-Separately-Excited  Dynamo. 
(5  )  The  Shunt-and-Separately-Excited  Dynamo. 
(6.)  The  Series-and-Shuut-Dynamo,  or  the  Com- 
pound Dynamo. 

In  the  series  dynamo  the  circuits  of  the  field  mag- 


302  ELECTRICAL  MEASUREMENTS. 

nets  and  the  external  circuit  are  connected  in  series 
with  the  armature  circuit,  so  that  the  entire  arma- 
ture current  passes  through  the  field  magnet  coils. 

Such  a  dynamo  is  shown  in  Fig.  143. 

In  the  series  dynamo  any  increase  in  the  resist- 
ance of  the  external  circuit  will  decrease  the  power 


D     D    o    o 
FIG.  144.— SHUNT  DYNAMO. 

of  the  machine  to  produce  current  on  account  of  the 
decrease  in  the  current  of  the  field  magnet  coils. 

In  the  same  way  a  decrease  in  the  resistance  of  the 
external  circuit  will  increase  the  power  of  the  ma- 
chine to  produce  current  from  the  increase  in  the  mag. 


DYNAMO-ELECTRIC  MACHINES.  303 

netizing  current.  In  practice  these  difficulties  are 
avoided  by  means  of  automatic  or  hand  regulators. 

In  the  shunt  dynamo,  shown  in  Fig.  144,  the  field 
magnet  coils  are  placed  in  a  shunt  to  the  external 
circuit,  so  that  a  portion  only  of  the  current  gener- 
ated passes  through  the  field  magnet  coils,  but  all 
the  difference  of  potential  of  the  armature  acts  at 
the  terminals  of  the  field  circuit. 

In  the  shunt  dynamo  any  increase  in  the  resistance 
of  the  external  circuit  causes  a  smaller  proportion 
of  current  to  pass  momentarily  in  the  external  cir- 
cuit and  a  larger  proportion  to  pass  in  the  field 
magnet  circuit,  and  the,  resulting  increase  in  the 
magnetism  causes  an  increase  in  the  current  pro- 
duced in  the  armature.  On  a  decrease  in  the  exter- 
nal resistance,  the  reverse  effects  follow.  A  properly 
proportioned  shunt  dynamo  will  therefore  be  self- 
regulating. 

When  the  armature  of  either  a  series  or  a  shun  I 
dynamo  begins  to  rotate,  the  current  produced  in 
its  coils,  under  the  influence  of  the  weak  residual 
magnetism  of  the  field  magnets,  passing  through 
the  magnetizing  coils  of  the  field  magnets,  increases 
the  magnetic  intensity  of  the  machine,  and,  thus  re- 
acting on  the  armature,  causes  a  more  powerful  cur- 
rent to  flow  through  the  field  magnet  coils.  This 


304  ELECTRICAL  MEASUREMENTS. 

again  increases  the  strength  of  the,  magnetic  field, 
and  again  reacts  to  increase  the  current  strength  in 
the  armature  coils,  and  the  action  continues  as  the 
machine  thus,  as  is  technically  said,  "builds  up" 
until  it  produces  its  full  output. 


PIG.  145.— SEPARATELY-EXCITED  DYNAMO. 

This  action  is  called  the  reaction  principle  of  the 
dynamo,  and  was  first  discovered  by  Soren  Hjorth, 
of  Copenhagen,  and  afterward  rediscovered  inde- 
pendently by  Siemens  and  Wheatstone. 

In  the  separately-excited  machine,  shown  in  Fig. 
145,  the  field  magnet  coils  have  no  connection  with 


DYNAMO-ELECTRIC  MACHINES.  305 

the  armature  coils,  but  receive  their  current  from  a 
separate  machine  or  other  source. 

In  the  series-and-separately-excited  dynamo  elec- 
tric machine,  as  shown  in  Fig.  146,  the  field  magnet 
cores  are  wound  with  two  separate  coils,  one  of  which 
is  connected  in  series  with  the  armature  and  the  ex- 


FIG.  146.— SERIES-AND  SEPARATELY-EXCITED  DYXAMO. 
ternal  circuit,  and  the  other  with  some  source  exter- 
nal to  the  machine  by  means  of  which  it  is  separately 
excited.  Since  in  these  machines  the  field  magnet 
cores  have  two  separate  and  independent  coils  wound 
on  them  they  are  generally  called  compound-wound 
machines. 


306 


ELECTRICAL  MEASUREMENTS. 


The  shunt-and-separately-excited  dynamo  shown 
in  Fig.  147  is  also  a  compound  wound  machine  ;  for, 
it  has  two  independent  sets  of  coils  in  the  cores  of 
its  field  magnets.  In  this  type  of  compound-wound 
dynamo  electric  machine  the  field  is  excited  both  by 


FIG.  147.— SHUNT- AND-SEPARATELY-EXOITED  DYNAMO. 
means  'of  a  shunt  to  the  external  circuit  and  by 
means  of  the  current  produced  by  a  separate  source. 
A  series-and-shunt-wound  dynamo  is  shown  in  Fig. 
148.  In  this  machine  the  field  magnet  cores  are 
wound  with  two  separate  coils,  one  of  which  is 
placed  in  series  with  the  armature  circuit  and  the 
other  in  shunt  to  the  external  circuit. 


DYNAMO-ELECTRIC  MACHINES. 


307 


The  series-and-shunt-wound  dynamo  electric  ma- 
chine is  generally  employed  to  maintain  a  constant 
difference  of  potential  at  its  terminals.  In  some 
forms,  however,  the  machines  are  over-compounded 
so  as  to  increase  their  electro-motive  force  on  an  in- 


Fio.  U8.— SERIBS-AND-SHTJNT  WOUND  DYNAMO. 

crease  of  current.  It  is  some  variety  of  the  series- 
and-shunt-wound  dynamos  that  is  generally  used 
commercially.  It  is  generally  known  as  the  com- 
pound machine,  or  compound-wound  machine. 

The  great  value  of  the  use  of  the  compound  or 
differentially- wound  dynamo  electric  machine,  in  sys- 


308  ELECTRICAL  MEASUREMENTS. 

terns  of  incandescent  electric  light  distribution,  in 
maintaining  automatically  a  practically  constant 
difference  of  potential  on  the  mains,  will  be  under- 
stood when  it  is  remembered  that  any  considerable 
variation  in  the  difference  of  potential  would  render 
the  commercial  use  of  such  lights  impracticable  by 
reason  of  their  unsteadiness 


DYNAMO  ELECTRIC  MACHINES. 


EXTRACTS  FROM  STANDARD  WORKS. 

Silvanus  P.  Thompson  in  his  Third  Edition  of 
"  Dynamo-Electric  Machinery,"*  in  the  introduction 
to  the  physical  theory  of  the  dynamo  says,  page  33  : 


A  very  large  number  of  dynamo-electric  machines  have 
been  constructed  upon  the  foregoing  principles.  The  variety 
is,  indeed,  so  great  that  classification  is  not  altogether  easy. 
Some  have  attempted  to  classify  dynamos  according  to 
some  constructional  points,  such  as  whether  the  machine 
did  or  did  not  contain  iron  in  its  moving  parts  (which  is 
mere  accident  of  manufacture,  since  almost  all  dynamos 
will  work,  though  not  equally  we!l,  either  with  or  without 
iron  in  their  armatures) ;  or  whether  the  currents  gener- 
ated were  direct  and  continuous,  or  alternating  (which  is 
in  many  cases  a  mere  question  of  arrangement  of  parts  of 
the  commutators  or  collectors) ;  or  what  was  the  form  of 
the  rotating  armature  (which  is,  again,  a  matter  of  choice 
in  construction,  rather  than  of  fundamental  principle). 

Suppose,  then,  it  was  determined  to  construct  a  dynamo 
upon  any  one  of  these  plans — say  the  first — a  very  slight 

•"Dynamo-Electric  Machinery:  A  Manual  for  Students  of  Electro- 
technics,"  by  Silvanua  P.  Thompson,  D.  Sc..  B.  A.,  F.  R.  S.  Third 
ediiion,  enlarged  and  revised.  London:  E.  &  F.  NT.  Spon,  1888. 
h  our  th  edition.  1892.864  pages,  498  illustrations,  29  plates.  Price, 
$9.00. 


310  ELECTRICAL  MEASUREMENTS. 

acquaintance  with  Faraday's  principle  and  its  corollaries 
would  suggest  that,  to  obtain  powerful  electric  currents, 
the  machine  must  be  constructed  upon  the  following  guid- 
ing lines: 

(a.)  The  field  magnets  should  be  as  strong  as  possible,  and 
their  poles  as  near  to  the  ai mature  as  possible. 

(&.)  The  armature  should  have  the  greatest  possible 
length  of  wire  upon  its  coils. 

(c.)  The  wire  of  the  armature  coils  should  be  as  thick  as 
possible,  so  as  to  offer  little  resistance. 

(d.)  A  very  powerful  steam  engine  should  be  used  to  turn 
the  armature,  because, 

(e.)  The  speed  of  rotation  should  be  as  great  as  possible. 

Unfortunately,  it  is  impossible  to  realize  all  these  con- 
ditions at  once,  as  they  are  incompatible  with  one  another ; 
and,  moreover,  there  are  a  great  many  additional  con- 
ditions to  be  observed  in  the  construction  of  a  successful 
dynamo.  We  will  deal  with  the  various  matters  in  order, 
beginning  with  the  various  organs  or  parts  of  the  machine. 
Having  discussed  these,  we  take  up  the  nature  of  the  proc- 
esses that  go  on  in  the  machine  when  it  is  at  work,  the 
action  of  the  magnetic  field  on  the  rotating  armature,  the 
reactions  of  the  armature  upon  the  field  in  which  it  ro- 
tates, and  the  various  methods  of  exciting  and  governing 
tie  magnetism  of  the  field  magnets.  After  that  we  shall 
be  in  a  position  to  enter  upon  the  various  actual  types  of 
machines  for  generating  continuous  and  alternating  cur- 
rents. 


XV.— ELECTRO-DYNAMICS. 


Electro-dynamics  is  that  branch  of  electricity 
which  treats  of  the  action  of  an  electric  current  on 
itself,  on  another  current,  or  on  a  magnet. 

The  term  "  electro-dynamics"  is  employed  in  con- 
tradistinction to  electro-statics.  Electro-dynamics 
treats  of  the  effects  produced  by  electricity  in  motion. 
Electro-statics  treats  of  the  effects  produced  by 
electricity  at  rest. 

Shortly  after  Oersted  discovered  the  relation  exist- 
ing between  electricity  and  magnetism,  Ampere  in- 
vestigated the  action  which  neighboring  circuits 
exert  on  one  another,  when  electric  currents  are 
flowing  through  them.  After  an  extended  series  of 
investigations  he  announced  the  following  laws1. 

(1.)  Parallel  circuits,  through  which  electric 
currents  are  flowing  in  the  same  direction,  attract 
each  other. 

(2.)  Parallel  circuits,  through  which  electric  cur- 
rents are  flowing  in  opposite  directions,  repel  each 
other. 

(3.)  Circuits  whose  pkines  intersect,  mutually  at- 
(311) 


313 


ELECTRICAL  MEASUREMENTS. 


tract  each  other  when  the  currents  through  them 
flow  either  toward  or  from  the  point  of  intersection. 

(4.)  Circuits  whose  planes  intersect,  mutually  re- 
pel each  other  when  the  currents  through  them  flow 
so  that  one  approaches  and  the  other  recedes  from 
the  point  of 'intersection. 

The  correctness  of  these  laws  can  be  determined 
experimentally  by  a  variety  of  apparatus. 

In  the  apparatus  shown  in  Fig.  149,  metallic  pillars 


FIG.  H9.— DEFLECTION  OP  A  CIRCUIT  BY  A  CURRENT. 
B,  B',  support  horizontal  metallic  arms  that  are  pro- 
vided at  y  and  c,  with  mercury  cups  situated  above 
one  another  in  the  same  vertical  line,  for  the  inser- 
tion of  the  ends  of  a  rectangular  circuit  C  C',  of 
conducting  wire  bent  at  its  upper  extremity  and  pro- 
vided with  points  extending  downward.  When 
these  points  are  supported  in  mercury  cups  at  y  and 


ELECTRO-DYNAMICS.  313 

c,  the  rectangular  circuit  is  suspended  as  shown  in 
the  figure. 

In  this  apparatus,  as  in  most  of  the  apparatus  em- 
ployed to  demonstrate  Ampere's  laws,  a  circuit  is 
provided  consisting  of  two  parts  ;  namely,  a  fixed 
circuit  B  B',  and  a  movable  circuit  C  C'.  When  the 
terminals  of  an  electric  source  are  connected  to  the 
points  marked  +  and  — ,  an  electric  current  flows 
up  the  pillar  marked  B',  and  down  the  end  C' ,  of 
the  circuit  nearest  thereto,  returns  up  the  left 
hand  side  of  the  circuit,  and  returns  to  the  source 
down  the  outer  pillar  B. 

Under  these  circumstances,  if  the  plane  of  the 
movable  circuit  C  C',  coincides  with  the  plane  of  the 
fixed  circuit  B  B',  when  the  current  is  ca'used  to 
flow  through  it  by  the  connection  as  shown,  C  C', 
will  be  repelled  because  the  current  in  the  branch 
of  the  circuit  B',  is  flowing  in  the  opposite  direction 
to  the  current  in  the  branch  C'. 

A  more  convenient  way  of  showing  these  move- 
ments is  by  means  of  the  apparatus  represented  in 
Fig.  150.  Here  the  fixed  circuit  is  given  the  form 
of  a  coil  of  insulated  wire  M  N,  and  the  movable 
circuit  B  C,  is  supported  at  points  above  and  below, 
as  shown,  so  as  to  be  free  to  move.  When  the  con- 
nections are  made  as  indicated  in  the  figure,  the  di- 


314 


ELECTRICAL  MEASUREMENTS, 


rection  of  the  current   through  the  two  circuits  is 
as  represented  by  the  arrows. 

Supposing,  now,  that  before  the  current  is  passed, 
the  movable  circuit  has  been  placed  in  the  same 
plane  as  that  of  the  coil  M  N ;  then,  since  the  branch 
M,  of  the  fixed  circuit,  has  a  current  flowing  through 
it  in  the  opposite  direction  to  the  branch  B,  of  the 
movable  circuit,  as  soon  as  such  current  begins  to  flow 


FIG.  150.— AMPERE'S  STAND. 

a  repulsion  occurs,  and  the  movable  circuit  tends  to 
place  itself  with  its  plane  at  right  angles  to  that  of 
the  fixed  circuit,  and  will  so  place  itself,  provided 
the  current  is  sufficiently  strong.  If,  however,  the 
direction  of  the  current  in  the  branch  M,  of  the  fixed 
circuit  is  the  same  as  that  in  the  branch  B,  of  the 
movable  circuit  (which  can  be  effected  by  reversing 
the  position  of  the  fixed  circuit  M  N),  on  the  passage 


ELECTRO-DYNAMICS.  315 

of  the  current,  if  the  movable  coil  has  been  previ- 
ously placed  so  that  its  plane  does  not  coincide  with 
the  plane  of  the  fixed  coil,  it  will  be  attracted 
thereto. 

In  order  to  demonstrate  the  third  law,  the  appa- 
ratus shown  in  Fig.  151  may  be. employed.  Here  the 
movable  circuit  A  B  C  D,  supported  as  shown, 
and  having  a  current  flowing  through  it  in  the 


FIG.  151.— ATTRACTION  OF  ANGULAR  CURRENTS. 

direction  indicated  by  the  arrows,  will  be  attracted 
by  the  fixed  circuit  Q  P,  when  the  current  in  it 
flows  in  the  direction  from  Q  to  P,  for  then  the  cur- 
rents in  both  the  fixed  and  movable  circuits  are  flow- 
ing in  the  same  direction  both  toward  and  from  Y, 
the  point  of  intersection  of  the  circuits. 
If,  however,  the  current  flows  through  the  fixed 


316  ELECTRICAL  MEASUREMENTS. 

circuit  in  the  opposite  direction,  or  from  P  to  Q, 
then  since  the  currents  are  flowing  in  opposite 
directions  in  the  fixed  and  movable  circuit  toward 
and  from  the  point  of  intersection  Y,  the  fixed  cir- 
cuit will  repel  the  movable  circuit. 

The  fact  that  parallel  currents  flowing  in  the  same 
direction  attract  each  other  can  be  shown  by  the  fol- 
lowing experiment  :  When  an  electric  current  flows 
through  a  flexible  conductor  wound  in  the  shape  of  a 
loose  or  open  spiral,  the  current  flows  in  the  same 
direction  through  the  contiguous  turns  of  the  spiral 
and  the  mutual  attractions  exerted  between  such 
turns  cause  them  to  attract  one  another,  thus 
shortening  the  spiral.  If  the  spiral  conductor  is 
supported  at  its  upper  end,  so  that  its  lower  end  dips 
into  a  mercury  surface,  and  the  current  flowing 
through  the  spiral  is  made  sufficiently  strong,  the 
mutual  attractions  will  shorten  the  spiral  sufficiently 
to  cause  it  to  lift  itself  out  of  the  mercury  surface 
and  thus  break  the  circuit.  Th'e  spiral  then  falls 
and  again  makes  contact  with  the  mercury  surface 
and  causes  the  current  to  again  flow  through  it. 
There  is  thus  established  a  succession  of  automatic 
makes-and-breaks  of  the  circuit  that  follow  one 
another  with  considerable  rapidity.  A  brilliant 
spark,  caused  by  the  induced  current  on  breaking 


ELECTRODYNA  MICS. 


31? 


the  circuit,  appears  each  time  the  spiral  leaves  the 
surface  of  the  mercury. 

Electro-dynamic  attractions  and  repulsions  are 
also  produced  by  the  action  of  magnets  on  movable 
circuits.  This  should  be  expected,  since  electric 
circuits  possess  all  the  properties  of  magnets. 

If  the  magnet  A  B,  be  placed  parallel  to  the  mov- 
able circuit  shown  in  Fig.  152,  in  the  direction 


FIG.  152.— DEFLECTION  OF  CIRCUIT  BY  A  MAGNET. 
shown  by  the  dotted  lines,  and  the  current  be  then 
passed  through  the  movable  circuit  G  C,  in  the  di- 
rection indicated  by  the  arrows,  the  circuit  will  move 
and  tend  to  place  itself  at  right  angles  to  the  axis 
of  the  magnet,  or  in  the  position  shewn  by  the  full 
lines  in  the  figure.  A  careful  study  of  these  move- 
ments will  show  that  they  are  the  same  as  would  oc- 


318  ELECTRICAL  MEASUREMENTS. 

cur  if  an  electric  current  were  circulating  around 
the  magnet  in  the  same  direction  as  the  currents 
which  Ampere  assumed  to  exist  in  all  magnets  and 
to  be  the  cause  of  their  magnetism. 

It  is  a  well-known  fact  that  when  a  current  flows 
through  the  coils  of  a  solenoid,  that  is,  of  a  cylin- 
drical coil  of  wire  the  convolutions  of  which  are  cir- 
cular, the  solenoid  acquires  all  the  properties  of  a 
magnet,  so  that  it  will  be  attracted  or  repelled  by 
another  magnet,  or  by  another  solenoid  placed  near  it, 
through  whose  circuit  an  electric  current  is  flowing. 

As  in  the  case  of  magnetic  attractions  and  repul- 
sions, like  solenoidal  poles  repel  and  unlike  poles  at- 
tract. This  action  of  solenoids  on  one  another  will 
be  understood  from  a  consideration  of  the  directions 
of  the  currents  required  to  produce  the  respective 
north  or  south  poles. 

Unlike  poles  of  a  solenoid  attract  each  other  be- 
cause the  currents  which  produce  such  poles  flow 
in  the  same  direction  in  parts  of  the  circuits  which 
lie  nearest  to  each  other. 

In  the  same  manner  like  poles  of  a  solenoid  repel 
each  other,  because  the  currents  which  produce  such 
poles  flow  in  the  opposite  direction,  in  parts  of  the 
circuit  which  lie  nearest  each  other. 

The  cause  of  the  attractions  which  exist  between 


ELECTRO-DYNA  MICS. 


319 


the  unlike  poles  of  solenoids  is  to  be  found  in  the 
direction  of  the  lines  of  magnetic  force  which  are 
produced  by  the  currents  flowing  through  such  solen- 
oids. 

Like  charges  of  electricity  and  like  magnet  poles 
repel,  and  there  is,  therefore,  to  many  students  a 
difficulty  in  understanding  why  parallel  currents 
flowing  in  the  same.direction  in  neighboring  circuits 
should  attract  each  other,  but  the  reason  is  apparent 


FIG.  153.— MUTUAL  ACTION  OF  MAGNKTIC  FIELDS. 
when  traced  to  the  magnetic  fields  which  such    cur- 
rents produce. 

The  cause  of  this  will  be  understood  from  a  study 
of  Fig.  153,  which  shows  on  the  right  hand  two 
circuits  extending  parallel  to  each  other  through 
which  currents  are  flowing  in  opposite  directions, 
and  on  the  left  hand  side  two  parallel  circuits 
through  which  currents  are  flowing  in  the  same  di- 


320  ELECTRICAL  MEASUREMENTS. 

rection.  The  small  arrows  show  the  directions  of 
the  lines  of  magnetic  force  produced  by  the  current. 

Tracing  the  direction  of  the  circular  lines  of 
magnetic  force,  Avhich  are  produced  by  the  currents 
flowing  through  the  circuit  on  the  left,  it  will  be 
noticed  that  their  lines  of  magnetic  force  in  those 
parts  of  the  circuit  where  the  lines  lie  nearest  to  one 
another  extend  in  opposite  direction. 

Parallel  circuits,  therefore,  flowing  in  the  same 
direction,  attract  one  another  because  their  ap- 
proached lines  of  magnetic  force  extend  in  oppo- 
site directions,  and  oppositely  directed  lines  of  mag- 
netic force  attract  one  another. 

Similarly,  if  the  drawing  at  the  right  hand  be  in- 
spected, which  represents  the  magnetic  fields  pro- 
duced by  two  parallel  circuits,  the  currents  through 
each  of  which  are  flowing  in  opposite  directions,  it 
will  be  seen  that  their  lines  of  force  have  the  same 
direction  in  parts  of  their  circuits  which  lie  nearest 
together,  and  that  these  lines  of  force  extending  in 
the  same  direction  repel  one  another. 

Parallel  currents,  therefore,  flowing  in  opposite 
directions  repel  one  another,  because  their  ap- 
proached lines  of  magnetic  force  extend  in  the  same 
direction. 

The  mutual  action,  therefore,  of  parallel  currents 


ELECTRO-DYNA  MICS. 


321 


is  thus  to  be  traced  to  the  action  of  the  magnetic 
lines  of  force  produced  by  the  currents. 

Generally  these  laws  may  be  expressed  as  follows  : 
Lines  of  magnetic  force  extending  in  opposite 
directions  attract  one  another  ;  lines  of  magnetic 
force  extending  in  the  same  direction  repel  one  an- 
other. 


FIG.  151.— RECTILINEAR  EQUIVALENT  OF  SINUOUS  CURRENT. 

Ampere  proved  that  when  a  circuit  is  bent  on  it- 
self, so  that  the  current  flows  in  one  part  of  the  cir- 
cuit in  the  opposite  direction  to  that  in  which  it 
flows  in  the  remainder  of  the  circuit,  the  two 
parts  exert  no  force  of  magnetic  attractions  or  re- 
pulsions on  external  objects.  This  expedient  of 
doubling  a  wire  on  itself  is  adopted  in  the  manu- 


322  ELECTRICAL  MEASUREMENTS. 

facture  of  resistance  coils,  in  order  to  prevent  the 
magnetic  fields  produced  by  such  coils  exerting  any 
disturbance  on  the  needle  of  the  galvanometer. 

If  a  circuit  is  bent,  as  shown  to  the  right  in  Fig. 
154,  so  that  one  part  of  it,  as  at  B' ,  is  formed  of  a 
straight  conductor  and  the  other  portion,  as  A',  is 
formed  of  a  zig-zag  conductor,  although  the  portion 
A',  is  longer  than  the  portion  B',  yet  if  a  current  be 
sent  into  such  a  wire  at  one  end  and  passed  out  at 
the  other  end  it  will  produce  no  action  on  the 
movable  circuit  ABC,  when  approached  to  it,  be- 
cause the  current  flowing  through  the  branch  B', 
neutralizes  the  effect  of  the  current  flowing  through 
the  branch  A'. 

The  term  sinuous  current  is  sometimes  applied 
to  a  current  flowing  through  a  sinuous  conductor. 

Successive  portions  of  the  same  rectilinear  current 
repel  one  another.  In  other  words,  a  current  flow- 
ing through  one  part  of  a  straight  or  rectilinear 
conductor  tends  to  repel  the  part  lying  nearest  to  it 
and  is  itself  repelled. 

A  circuit  0  A,  Fig.  155,  movable  around  0,  as  a 
centre,  will  be  continuously  rotated  in  the  direction 
shown  by  the  curved  arrow  by  the  current  flowing 
through  the  rectilinear  circuit  Q  P,  in  the  direction 
shown.  If  the  direction  of  the  current  through  the 


ELECTRODYNAMICS.  823 

movable  circuit  be  as  indicated  by  the  smaller  arrows, 
there  will  be  attraction  in  the  positions  correspond- 
ing to  (1)  and  (2),  and  repulsion  in  the  position  cor- 
responding to  (4). 

A  conductor  through  which  a  current  of  electric- 
ity is  flowing  tends  to  rotate  continuously  around  a 
magnet  pole,  as  can  be  shown  by  suitably  mounting 
a  conductor  so  as  to  be  capable  of  rotation  around  a 
magnet.  When  a  current  of  electricity  is  sent 
through  such  a  conductor,  the  conductor  will  ro- 


P  Q 

Fid.  155.— CONTINUOUS  ROTATION  OP  CURRENT. 

tate  continuously  in  one  direction  arou-nd  the  north 
magnetic  pole,  and  in  the  opposite  direction  around 
the  south  magnetic  pole. 

By  reason  of  the  mutual  actions  exerted  between  a 
conductor  through  which  a  current  of  electricity  is 
flowing  and  a  magnet  pole  the  circuit  tends  to  wrap 
or  twist  itself  around  such  pole,  aa  can  be  shown 
by  the  following  experiment  suggested  by  Lodge  : 

If  a  powerful   current  of    electricity  ia  passed 


324  ELECTRICAL  MEASUREMENTS. 

through  some  eight  feet  in  length  of  gold  thread, 
such  as  is  employed  for  making  gold  lace,  hung  in 
a  vertical  position  near  a  vertical  bar  magnet,  as 
soon  as  the  current  passes  through  the  thread  it  will 
wrap  itself  around  the  bar  magnet,  one-half  twisting 
itself  around  the  north  pole,  the  other  half  around 
the  south  pole. 

Or,  the  same  thing  can  be  shown  by  the  following 
experiment  suggested  by  S.  P.  Thompson: 

A  stream  of  mercury,  which  is  falling  between 
the  poles  of  a  powerful  electro-magnet,  will,  when 
an  electric  current  is  flowing  through  the  coils  of 
the  magnet  so  as  to  energize  it,  be  twisted  in  a 
spiral  direction,  which  will  vary  both  with  the  direc- 
tion of  the  current  or  with  the  magnetic  polarity. 


ELECTRO  &YNAMICS.  325 

EXTRACTS  FROM  STANDARD  WORKS. 

In  the  revised  edition  of  the  "  Student's  Text- 
Book  of  Electricity,"*  by  Noad,  on  page  264,  the  fol- 
lowing statements  are  made  concerning  the  action  of 
circuits  through  which  electric  currents  are  passing 
on  magnets: 

The  grand  fundamental  fact  observed  by  Oersted  in  1819 
was  that  when  a  magnetic  needle  is  brought  near  the  con- 
necting medium  (whether  a  metallic  wire  or  charcoal,  or 
even  saline  fluids)  of  a  closed  voltaic  circuit,  it  is  immedi- 
ately deflected  from  its  position,  and  made  to  take  up  a 
new  one,  depending  on  the  relative  positions  of  the  needle 
and  conductor. 

The  extent  of  the  declination  of  the  needle  depends  en- 
tirely on  the  quantity  of  electricity  passing  along  the  con- 
ductor ;  it  has  nothing  to  do  with  its  tension,  which  is  prob- 
ably the  reason  that  the  first  inquirers  failed  to  discover  the 
above  effects  since  they  all  worked  with  statical  electricity. 

When  the  current  is  rectilinear,  the  length  of  the  con- 
ducting wire  considerable,  so  that  in  relation  to  that  of  the 
needle  it  may  be  regarded  as  infinite,  the  intensity  of  the 
electro -magnetic  force  was  shown  by  Biot  and  Savary  to  be 
"in  the  inverse  ratio  to  the  simple  distance  of  the  magnetized 

*"The  Student's  Text-Book  of  Electricity,"  by  Henry  M.  Noad, 
Ph.  D  ,  F.R.S.  Revised  by  W.  H.  Preece,  M.I.C.E.  London:  Crosby 
Lockwood  &  Co.  1879.  615  pages%  471  illustrations.  Price  ?4. 00. 


326  ELECTRICAL  MEASUREMENTS. 

needle  from  the  current;"  but  it  is  only  under  these  conditions 
that  the  law  is  true,  for  it  has  been  shown  by  Laplace  that 
the  elementary  magnetic  force— that  is,  the  elementary  ac- 
tion of  a  simple  section  of  the  current  upon  the  needle — is, 
like  all  other  known  forces,  in  the  inverse  ratio  of  the  square 
of  the  distance,  and  proportional  to  the  sine  of  the  angle 
formed  by  the  direction  of  the  current,  and  by  the  line  drawn 
through  the  centre  of  the  section  to  the  centre  of  the  needle. 
In  fact,  by  calculating,  according  to  this  principle  the  sum  of 
all  the  elementary  actions  that  are  exercised  on  a  small  needle 
by  an  indefinite  rectilinear  current,  it  is  found  that  the  in- 
tensity of  this  resultant  should  be,  as  experiment  proves  it 
really  is,  in  the  inverse  simple  ratio  of  the  distance. 


XVI.—THE  ELECTRIC  MOTOR. 


An  electric  motor  consists  of  any  combination  of 
parts  by  means  of  which  electrical  energy  is  con- 
verted into  mechanical  energy. 

In  electric  motors,  as  now  generally  constructed, 
the  electrical  energy  is  caused  to  produce  mechanical 
energy  by  means  of  the  attractions  and  repulsions 
\vhich  are  exerted  between  the  magnetic  fields  of 
electro-magnets,  or  between  the  magnetic  fields  of 
electro-magnets  and  the  magnetic  fields  produced 
by  currents  flowing  through  neighboring  conduct- 
ors. 

An  electro-magnetic  motor  depends  for  its  opera- 
tion on  the  tendency  of  a  conductor  through  which 
a  current  of  electricity  is  flowing  to  move  in  a  mag- 
netic field  in  accordance  with  the  principles  of 
electro-dynamics  already  pointed  out.  Such  a  tend- 
ency to  motion  arises  from  the  magnetic  attractions 
and  repulsions  which  the  two  fields  exert  on  each 
other.  The  entire  magnetism  may  be  produced  by 
a  current,  or  part  by  a  current,  and  the  rest  by  the 
field  of  a  permanent  magnet.  In  actual  practice, 

however,  electro-magnets  are  generally  employed. 
(327) 


338  ELECTRICAL  MEASUREMENTS. 

One  of  the  simplest  and  earliest  forms  of  electric 
motors  was  that  in  which  the  motion  was  obtained 
from  the  interactions  existing  between  the  magnetic 
field  of  a  conductor  through  which  a  current  of 
electricity  is  passing,  and  that  produced  by  the  poles 
of  a  permanent  magnet.  Such  a  form  of  apparatus, 
which  was  known  as  Barlow's  wheel,  is  shown  in 
Fig.  156.  Here  a  metallic  wheel  rotates  in  the  mag- 
netic field  produced  by  the  poles  of  a  permanent 
horseshoe  magnet,  in  the  direction  shown  by  the 


FIG.  156.— BARLOW'S  WHEEL. 

curved  arrow,  when  a  current  of  electricity  is  sent 
through  it  from  the  axis  to  the  'circumference. 

A  dynamo-electric  machine  is  capable  of  acting  as 
a  motor  if  a  current  of  electricity  is  sent  through 
its  circuit.  In  point  of  fact  the  construction  of  the 
modern  electric  motor  in  general  is  based  on  the  same 
principles  as  the  construction  of  dynamo-electric 
machines. 

The  discovery  of  the  reversibility  of  a  dynamo- 
electric  machine,  or  its  ability  to  operate  as  a  motor 


THE  ELECTRIC  MOTOR.  329 

when  a  current  of  electricity  is  passed  through  it 
may  be  said  to  have  been  the  discovery  on  which  the 
introduction  of  the  electric  motor,  as  it  exists  at  the 
present  day,  is  based. 

The  following  analogies  can  be  shown  to  exist  be- 
tween dynamo-electric  machines  and  electric  motors: 

(1.)  If  mechanical  energy  is  applied  to  a  dynamo- 
electric  machine,  so  that  its  armature  is  caused  to 
rotate,  a  difference  of  electromotive  force  is  pro- 
duced in  such  armature  ;  and,  conversely,  if  electric 
energy  be  applied  to  a  dynamo-electric  machine,  by 
sending  a  current  through  its  circuit,  the  armature 
will  rotate  and  produce  mechanical  energy. 

(2.)  If  mechanical  energy  is  applied  to  a  prop- 
erly designed  dynamo-electric  machine  such,  for  ex- 
ample, as  that  made  for  incandescent  lighting,  so  that 
its  armature  is  run  at  a  constant  speed,  the  electro- 
motive force  which  it  produces  will  remain  prac- 
tically constant  no  matter  what  load  may  be  on  the 
machine,  or  how  much  current  it  is  generating ; 
conversely,  if  electric  energy  is  applied  to  an  electric 
motor,  designed  so  as  to  have  a  constant  field,  by 
maintaining  a  constant  difference  of  potential  at  its 
terminals,  it  will  run  at  a  constant  speed  no  matter 
what  load  may  be  placed  on  it. 

In  an  electric  motor  the  pull  produced  along  the 


330  ELECTRICAL  MEASUREMENTS. 

circumference  of  the  shaft  by  the  electro- dynamic 
action  of  the  fields,  or,  in  other  words,  the  amount  of 
turning  force  which  such  shaft  exerts,  is  called  its 
torque.  In  a  well-constructed  motor,  as  the  load  on 
the  motor  increases,  the  torque  increases  proportion- 
ally. 

By  the  efficiency  of  an  electric  motor  is  meant  the 
ratio  which  exists  between  the  electric  energy  re- 
quired to  drive  the  motor  and  the  mechanical  energy 
which  it  gives  out. 

The  efficiency  of  an  electric  motor  may  be  made 
very  high ;  it  may  even  rise  to  not  far  from  100  per 
cent.  When,  however,  a  motor  is  overloaded  its 
efficiency  rapidly  decreases. 

When  mechanical  power  is  applied  to  drive  the 
armature  of  a  dynamo,  the  circuit  of  which  is  closed, 
an  electric  current  is  produced  which  flows  through 
the  circuit  of  the  machine  and  sets  up  in  its  field 
magnets  a  magnetic  polarity,  which  will  be  of  such 
a  character,  as  compared  with  that  produced  by  its 
armature,  that  the  armature  will  oppose  or  resist  be- 
ing moved  in  the  direction  in  which  it  is  moved  in 
order  to  produce  differences  of  potential. 

If,  however,  it  is  compelled  to  move  in  this  direc- 
tion past  the  field  magnet  poles,  the  mechanical 
energy  so  expended  is,  in  accordance  with  the  well- 


THE  ELECTRIC  MOTOR.  331 

known  principle  of  the  conservation  of  energy,  con- 
verted into  a  very  nearly  equal  amount  of  electrical 
energy  which  appears  in  the  current  flowing  through 
the  circuit  connected  therewith. 

Suppose,  for  example,  that  the  polarity  produced 
in  the  armature  is  of  such  a  character,  as  compared 
with  that  produced  in  the  field  magnets,  that  the 
poles  n  and  s,  are  produced  in  those  parts  of  the  ar- 
mature core  that  lie  in  the  vertical  gap  between  the 
poles  N  and  S,  of  the  field  magnet,  as  shown  in  Fig. 

157. 

n 


s 

FIG.  157.— POLARITY  OF  ARMATUBE  AND  FIELD. 

If  the  armature  is  moved  in  a  direction  such  that 
the  n,  pole  at  the  top  of  the  armature  core  moves 
toward  the  s,  pole  of  the  field  magnets,  that  is,  if  the 
top  of  the  armature  is  moved  toward  the  left  hand, 
no  energy  will  be  expended  against  the  magnetic  field 
and  no  current  will  be  produced.  If,  however,  the 
armature  is  driven  so  that  the  top  moves  from  left 
to  right,  then  the  north  pole  at  the  top  of  the  arma- 
ture is  moved  toward  the  north  pole  of  the  field 


332  ELECTRICAL  MEASUREMENTS. 

magnet,  and  the  south  pole  at  the  bottom  is  moved 
toward  the  south  pole  of  the  field  magnet,  and  the 
energy  so  expended  is  converted  into  electrical  energy. 

If,  now,  such  an  armature  be  supplied  with  an 
electric  current  from  some  source  outside  the  ma- 
chine, and  the  current  flows  through  the  field  mag- 
nets and  the  armature  in  such  a  direction  that  the 
polarity  remains  the  same  as  shown  in  Fig.  156,  it 
can  readily  be  seen  that  the  armature  will  turn  in 
the  opposite  direction  to  that  in  which  it  requires  to 
be  turned  by  power  in  order  to  produce  differences 
of  potential ;  for  now  the  north  pole  at  the  top  of 
the  armature  will  be  attracted  and  will  move  toward 
the  south  pole  of  the  field  magnets,  and  the  south 
pole  at  the  bottom  of  the  armature  will  be  attracted 
and  will  move  toward  the  north  pole  of  the  field 
magnets. 

A  difference  exists  between  the  position  the  col- 
lecting brushes,  or,  more  properly  speaking,  the 
distributing  brushes,  must  have  on  the  commutator 
cylinder  of  an  electric  motor  and  on  the  commu- 
tator cylinder  of  a  dynamo-electric  machine  in  order 
that  there  shall  be  no  excessive  sparking.  In  both 
cases  a  lead  must  be  given  to  the  brushes  on  ac- 
count of  the  reaction  which  exists  between  the 
fields  produced  by  the  field  magnets  and  that  pro- 


THE  ELECTRIC  MOTOR.  333 

duced  by  the  armature  core.  In  the  case  of  a  motor, 
according  to  S.  P.  Thompson,  the  direction  of  this 
lead  will  be  forward,  or  in  the  direction  of  rota- 
tion, if  it  is  only  desired  to  obtain  a  rapid  rotation. 
But,  if  it  is  desired  to  obtain  the  position  of  least- 
sparking  it  must  be  moved  a  short  distance  in  the 
opposite  direction.  In  other  words,  the  lead  which 
must  be  given  to  the  brushes  on  the  commutator  of 
a  motor  is  in  the  opposite  direction  to  that  which 
must  be  given  to  the  brushes  on  the  commutator  of 
a  dynamo. 

For  the  purpose  of  avoiding  too  great  a  lead  for 
distributing  brushes,  the  magnetism  generated  in  the 
field  magnets  should  be  made  great  as  compared 
with  that  generated  by  the  armature.  It  may  be  ob- 
served in  this  connection,  that  with  the  best  con- 
struction of  motors  the  necessity  for  any  lead  for 
the  brushes  becomes  very  small. 

In  actual  practice  it  is  often  necessary  to  be  able  to 
readily  change  the  direction  in  which  the  motor  is 
running.  Such  a  change  in  direction  can  be  ob- 
tained in  the  following  ways  : 

(1.)  By  reversing  the  direction  of  the  current 
through  the  armature. 

(2.)  By  reversing  the  direction  of  the  current 
through  the  field  magnets,  but  not  by  reversing  the 


334  ELECTRICAL  MEASUREMENTS. 

direction  of  the  current  through  the  armature  and 
the  field  magnets  simultaneously. 

The  reversing  of  the  direction  of  the  current  can 
be  effected  by  changing  the  position  of  the  dis- 
tributing brushes,  and  at  the  same  time  changing 
the  position  of  the  lead,  provided  the  motor  works 
under  a  sensible  lead. 

A  little  consideration  will  show  that  in  an  ordi- 
nary bi-polar  dynamo-electric  machine,  by  rotating 
the  brushes  through  180°,  less  an  amount  equal  to 
twice  the  angle  of  the  ordinary  lead  (because  the 
lead  must  then  be  in  the  opposite  direction  to  what 
it  formerly  was),  will  reverse  the  direction  of  the 
motor's  motion. 

Any  device,  therefore,  by  which  the  brushes  can 
be  readily  moved  through  such  a  distance  will  effect 
a  reversal  of  the  motion  of  the  motor. 

This  method  of  reversal  has  been  employed  by 
Reckenzaun  and  others.  Reckenzaun's  reversing 
gear  is  shown  in  Fig.  158.  In  it  there  are  two  pairs 
of  brushes,  each  pair  of  which  is  fixed  to  the  com- 
mon brush-holder  and  is  capable  of  turning  on  a 
pivot  and  of  being  moved  or  rotated  by  the  motion 
of  a  lever  connected  therewith. 

We  will  now  consider  some  peculiarities  concern- 
ing the  direction  of  the  motion  that  will  be  produced 


THE  ELECTRIC  MOTOR.  335 

when  an  electric  current  is  sent  through  the  circuit 
of  dynamo-electric  machines  of  different  types. 

A  series  dynamo  will  operate  as  a  motor  when  a 
current  is  sent  through  it  in  a  direction  opposite  to 
that  of  the  current  which  it  produces  when  in  oper- 
ation as  a  generator ;  the  polarity  of  the  field  is  re- 


FIG.  158.— REVERSING  GEAR  FOR  ELECTRIC  MOTORS. 

versed  and  the  dynamo  will  turn  as  a  motor  in  the 
opposite  direction  to  that  required  to  produce  the 
current.  If  the  current  is  reversed,  the  polarity  of 
both  the  field  and  the  armature  will  be  reversed,  and 
the  machine  will  still  rotate  as  a  motor  in  the  oppo- 


336  ELECTRICAL  MEASUREMENTS. 

site  direction  to  that  in  which  it  rotated  as  a  gen- 
erator. 

A  series  dynamo,  therefore,  always  rotates  as  a 
motor  in  a  direction  opposite  to  that  in  which  it  is 
rotated  as  a  generator,  unless  the  polarity  of  its  field 
magnets  or  its  armature  only  is  reversed,  when  it 
may  be  made  to  rotate  in  the  same  direction  as  it  is 
rotated  as  a  generator. 

A  shunt  dynamo  when  operated  as  a  motor  will 
turn  in  the  same  direction  as  that  in  which  it  is 
turned  as  a  generator  ;  for,  if  the  direction  of  the 
current  in  the  armature  is  the  same  as  in  the  gener- 
ator, that  in  the  shunt  is  reversed. 

A  separately-excited  dynamo  will  operate  as  a 
motor  when  a  current  is  sent  through  its  armature, 
and  will  always  turn  in  a  direction  opposite  to  that 
in  which  its  armature  requires  to  be  turned  in  order 
to  produce  a  current  in  the  same  direction  as  the 
driving  current. 

A  compound-wound  dynamo  when  operated  as  a 
motor  will  move  in  a  direction  opposite  to  that  of  its 
motion  as  a  generator  when  its  series  coils  are  more 
powerful  than  its  shunt  coils,  and  in  the  same  di- 
rection when  its  shunt  coils  are  more  powerful  than 
its  series  coils. 

If  a  galvanometer  is  placed  in  the  circuit  of  a 


THE  ELECTRIC  MOTOR.  337 

motor,  the  terminals  of  which  are  maintained  at  a 
constant  difference  of  potential,  and  the  armature  of 
the  motor  is  fixed  so  as  to  be  unable  to  move/  a 
certain  current  will  flow  through  the  circuit  of  the 
motor  as  may  be  determined  by  the  deflection  of  the 
galvanometer  needle.  If,  now,  the  armature  of  the 
motor  be  permitted  to  move,  it  will  be  found  that 
the  more  rapid  its  motion  becomes  the  smaller  will 
be  the  current  that  passes  through  the  circuit,  as 
will  be  seen  by  a  smaller  deflection  of  the  galvanom- 
eter needle. 

The  cause  of  this  is  as  follows  :  As  the  armature 
moves  through  the  field  of  the  machine,  its  coils  of 
wire  cut  the  lines  of  magnetic  force  and,  just  as  in  a 
dynamo,  will  have  differences  of  potential  generated 
iu  them,  which  are  opposed  to  the  difference  of  po- 
tential of  the  current  which  drives  the  motor.  In 
this  way  a  counter-electromotive  force  is  set  up, 
which  acts  like  a  resistance  to  oppose  the  passage  of 
the  driving  current  through  the  coils  of  the  motor. 
Therefore,  as  the  speed  of  the  motor  increases,  the 
strength  of  the  driving  current  becomes  less,  until, 
when  the  maximum  speed  is  reached,  very  little  cur- 
rent passes. 

When,  however,  a  load  is  placed  on  a  motor,  so  that 
it  is  caused  to  do  work,  its  speed  tends  to  decrease, 


338  ELECTRICAL  MEASUREMENTS. 

and  the  counter-electromotive  force  is  decreased,  thus 
permitting  a  greater  driving  current  to  pass  through 
'the  circuit.  In  this  way  a  motor  automatically 
regulates  the  current  required  to  drive  it.  For  this 
reason,  therefore,  electric  motors  are  very  economical 
in  operation,  provided  they  are  efficient  at  full  load. 
The  relations  between  the  power  required  to  drive 
the  generating  dynamo  and  that  produced  by  the 
electric  motor,  through  which  its  current  passes, 
are  such  that  the  maximum  work  per  second  is  done 
by  the  motor  when  it  runs  at  such  a  rate  that  the 
counter-electromotive  force  it  produces  is  half  that 
of  the  current  supplied  to  it.  The  maximum  rate 
of  work  of  an  electric  motor  is  therefore  done  when 
its  theoretical  efficiency  is  only  50  per  cent.  This, 
however,  must  be  carefully  distinguished  from  the 
maximum  efficiency  of  an  electric  motor.  A  max- 
imum efficiency  of  100  per  cent,  can  be  attained 
theoretically,  and  considerably  over  90  per  cent,  is 
attained  in  practice.  In  such  cases,  however,  the 
motor  is  doing  work  at  less  than  its  maximum  rate. 

An  efficiency  of  100  per  cent,  would  be  theoreti- 
cally reached  when  the  counter- electromotive  force 
of  the  motor  is  equal  to  that  of  the  source  supplying 
the  driving  current.  If  the  driving  machine  is  of 
the  same  size  and  type  as  the  motor,  the  two  ma- 


THE  ELECTRIC  MOTOR,  339 

chines  would  be  running  at  the  same  speed.  If, 
now,  a  load  is  put  on  the  motor  so  as  to  reduce  its 
speed,  and  thus  prevent  it  from  producing  a  coun- 
ter-electromotive force  of  more  than  90  pei  cent.,  its 
efficiency  will  be  about  90  per  cent.  In  such  a  case, 
therefore,  the  efficiency  is  represented  by  the  rela- 
tive speeds  at  which  the  generator  and  motor  are 
running. 

Jacobi's  law  of  maximum  effect,  namely,  that  an 
electric  motor  gives  its  maximum  work  when  it  is 
geared  to  run  at  a  speed  which  reduces  the  current 
to  half  the  strength  it  would  have  when  at  rest,  does 
not  apply  to  motors  in  practice  on  account  of  limi- 
tation of  current  carrying  capacity.  For  example, 
a  motor  of  nine  horse  power  and  90  per  cent,  efficiency 
loses  one  horse  power  in  heating  itself,  and  would  be 
injured  if  much  more  than  one  horse  power  were  con- 
verted into  heat  in  it.  If  run  according  to  Jacobi's 
law  half  of  a  greater  amount  than  nine  horse  power 
would  be  converted  into  heat  in  itself,  and  this  would 
overheat  it. 

If  the  current  from  an  alternating  current  dynamo 
is  sent  through  a  similar  alternating  current  dynamo 
brought  up  to  the  same  speed,  it  will  drive  it  as  a 
motor.  Most  alternating  current  motors,  however, 
possess  the  disadvantage  of  requiring  to  be  main- 


340  ELECTRICAL  MEASUREMENTS. 

tained  at  exactly  the  same  speed  as  that  of  the  driv- 
ing dynamo,  Avith  the  additional  disadvantage  that 
they  require  to  be  brought  up  to  exactly  this  speed 
before  the  driving  current  can  be  supplied  to  them  ; 
overloading  which  reduces  the  speed,  even  by  the 
smallest  amount,  will  therefore  stop  the  motor.  Con- 
siderable improvements,  however,  are  being  intro- 


PIG.  159.— THE  C.  &.  C.  MOTOR. 

duced   into  alternating,  current  motors,  by  which 
these  difficulties  are  almost  entirely  removed. 

In  Fig.  159  is  shown  a  form  of  electric  motor  suit- 
able for  small  work,  called  the  0.  &  C.  motor,  from 
the  initials  of  its  inventors,  Curtis  and  Crocker.  Its 
armature,  which  consists  of  a  ring-wound  core,  is 


THE  ELECTRIC  MOTOR. 


341 


completely  enclosed  so  as  to  protect  it  from  injury, 
either  from  dust  or  other  causes. 

In  a  compound-wound  motor  the  field  magnets 
have  two  windings  which  oppose  each  other,  so  that 
the  speed  remains  constant  no  matter  what  may  be 


Fio.  160.-THE  SPRAGUE  ELECTRIC  MOTOR. 

the  load.  In  compound-wound  motors  the  series  coils 
are  wound  differentially  to  the  shunt,  coil,  so  that 
one  tends  to  demagnetize  the  field  magnets,  while  the 
other  tends  to  magnetize  them. 


342 


ELECTRICAL  MEASUREMENTS. 


A  form  of  compound- wound  motor  is  shown  in 
Fig.  160.  It  is  one  of  the  forms  of  the  Sprague 
motor. 

In  a  curious  form  of  motor,  known  as  the  pyro- 
magnetic  motor,  the  motion  is  obtained  by  the  at- 


Fio  161.— EDISON  PYRO-MAGNETIC- MOTOR. 

traction  which  magnet  poles  exert  on  an  unequally 
heated  movable  disc  of  iron. 

The  intensity  of  magnetization  of  iron  decreases 
with  an  increase  of  temperature,  the  iron  losing  all 
its  magnetism  at  a  red  heat.  If,  therefore,  a  disc  of 
iron  so  placed  between  the  poles  of  a  magnet  as  to 


THE  ELECTRIC  MOTOR. 


343 


be  capable  of  rotation,  is  heated  at  a  part  which 
lies  nearer  one  pole  than  the  other,  it  will  be  caused 
to  rotate,  since  it  becomes  less  powerfully  magnetized 
at  the  heated  part. 

Such  a  form  of  motor  does  not  at  present  possess 
very  great  efficiency. 


FIQ.  162.— EDISON  PYRO-MAGNKTIC  MOTOR. 

A  form  of  pyro-magnetic  motor  devised  by  Edison 
is  shown  in  Fig.  161  in  vertical  section  and  in  Fig. 
162  in  elevation. 

A  movable  cylinder  of  iron  A,  formed  by  a  bunch 
of  small  iron  tubes,  is  heated  by  the  products  of 
combustion  of  a  fire  placed  beneath  them.  To  ren- 


344  ELECTRICAL  MEASUREMENTS. 

der  this  heating  local,  a  flat  screen  S,  is  placed  dis- 
symmetrically across  the  top  to  prevent  the  passage 
of  hot  air  through  the  portion  of  the  iron  tubes  so 
screened.  The  air  is  supplied  to  the  furnace  by 
passing  down  from  above  through  the  tubes  so 
screened  and  thereby  cools  them.  This  is  shown  in 
the  drawings,  the  direction  of  the  heated  and  cooling 
air  currents  being  indicated  by  the  arrows.  Supply- 
ing the  air  from  above  insures  a  more  rapid  cooling  of 
the  screened  portion  of  the  tubes. 


THE  ELECTRIC  MOTOR.  345 


EXTRACTS  FROM  STANDARD  WORKS. 

Concerning  the  reversibility  of  the  dynamo-electric 
machine,  Martin  and  Wetzler,  in  "The  Electric 
Motor  and  Its  Applications,"*  on  page  29,  speak  as 
follows : 

But  there  is  another  version  of  the  story,  told  by  M.  Hip- 
polyte  Fontaine  to  the  Societe  des  Anciens  Eleves  des  Ecoles 
Nationales  des  Arts  et  Metiers.  M.  Fontaine  claims  to 
have  actually  invented  or  discovered  the  electrical  trans- 
mission of  power,  as  will  be  seen  from  the  following  short 
extract  from  his  paper,  read  before  the  above-mentioned 
society: 

On  the  1st  of  May,  1873— that  is,  on  the  date  fixed  four 
years  previously  by  imperial  decree — the  Exhibition  in 
Vienna  was  formally  opened.  At  that  time  the  machinery 
hall  was  as  yet  incomplete,  and  remained  closed  to  the  pub- 
lic until  the  3d  of  June,  when  it  was  also  thrown  open.  I 
was  then  engaged  with  the  arrangement  of  a  series  of  ex- 
hibits, shown  for  the  first  time  in  public,  which  were  in- 
tended to  work  together,  or  separately,  as  desired.  There 
was  a  dynamo  machine  by  Gramme  for  electroplating, 
giving  a  current  of  400  amperes  at  25  volts,  and  a  magneto 
machine,  which  I  intended  to  work  as  a  motor  from  a  pri- 
mary battery,  or  from  a  Plante  accumulator,  to  demon- 

*'Tbe  Electric  Motor  and  Its  Applications,"  by  T.  C.  Martin  and 
Jos.  Wetzler.  With  an  appendix  on  The  Development  of  the  Electric 
Motor  since  1888,  by  Dr.  Louis  Bell.  New  York:  The  W.  J.  Johns- 
ton Company,  Ltd.  1893.  325  pages,  354  illustrations.  Price,  $3.00. 


346  ELECTRICAL  MEASUREMENTS. 

strate  the  reversibility  of  the  Gramme  dynamo.  There 
was  also  a  steam  engine  of  my  invention  heated  by  coke,  a 
domestic  motor  of  the  same  type  heated  by  gas,  a  centrif- 
ugal pump  placed  on  a  large  reservoir,  and  arranged  to 
feed  an  artificial  cascade,  and  numerous  other  exhibits. 
To  vary  the  experiments  I  proposed  to  show,  I  had  arranged 
the  pump  in  such  a  way  that  it  could  be  worked  either  by 
the  Gramme  magneto  machine  or  by  the  steam  engines 
(Fontaine). 

On  the  1st  of  June  it  was  announced  that  the  machinery 
hall  would  be  formally  opened  by  the  Emperor  at  10  A.  M. 
on  the  day  after  the  morrow.  Nothing  was  then  in  readi- 
ness, but  those  who  have  been  in  similar  situations  know 
how  much  can  be  got  into  order  in  the  space  of  48  hours 
just  before  the  opening  of  a  great  exhibition.  In  every  de- 
partment members  of  the  staff  with  an  army  of  workmen 
under  their  orders  were  busily  clearing  away  packing 
boxes  and  decorating  the  space  allotted  to  the  different 
nations.  These  gentlemen  visited  all  the  exhibits  in  order 
to  determine  which  of  them  should  be  selected  for  the 
special  notice  of  the  Emperor,  so  as  to  detain  him  as  long  as 
possible  among  the  exhibits  of  their  respective  countries. 

M.  Roullex-Duggage,  who  superintended  the  work  hi  the 
French  section,  asked  me  to  set  in  motion  all  the  machinery 
on  my  stand,  and  especially  the  two  Gramme  machines.  I 
set  about  at  once,  and  on  the  3d  of  June  I  had  the  satis- 
faction of  getting  the  large  Gramme  dynamo,  the  two 
engines  (Fontaine),  and  the  centrifugal  pump  to  work 
but  I  failed  to  get  the  motor  into  action  from  the  primary 
or  secondary  battery.  This  was  a  great  disappointment, 


THE  ELECTRIC  MOTOR.  347 

especially  as  it  prevented  my  showing  the  reversibility  of 
the  Gramme  machine.  I  was  puzzled  the  whole  of  the 
evening  and  the  whole  of  the  night  to  find  a  means  to  ac- 
complish my  object,  and  it  was  only  in  the  morning  of 
June  3,  a  few  hours  before  the  visit  of  the  Emperor,  that 
the  idea  struck  me  to  work  the  small  machine  by  means  of 
a  derived  circuit  from  the  large  machine.  Since  I  had 
no  leads  for  that  purpose,  I  applied  to  the  representa- 
tives of  Messrs.  Manhis,  of  Lyons,  who  were  kind 
enough  to  lend  me  250  metres  of  cable,  and 
when  I  saw  that  the  magneto  machine  was  not 
only  set  in  motion,  but  developed  so  much  power  as  to 
throw  the  water  from  the  pump  beyond  the  reservoir,  I 
added  more  cable  until  the  flow  of  water  became  normal. 
The  total  length  of  cable  in  circuit  was  then  over  two  kilo- 
metres. This  great  length  gave  the  idea  that  by  the  em- 
ployment of  two  Gramme  machines  it  would  be  possible  to 
transmit  mechanical  energy  to  great  distances.  I  spoke  of 
this  idea  to  various  people,  and  I  published  it  in  the  Revue 
Industrielle  in  1873,  and  subsequently  in  my  book  on  the 
Vienna  Exhibition.  The  publicity  thus  given  to  it  was  so 
great  that  I  had  neither  time  nor  desire  to  protect  my  in- 
vention by  a  patent.  I  must  also  mention  that  M.  Gramme 
has  told  me  that  he  had  already  worked  one  dynamo  by 
the  other,  and  I  have  always  held  that  the  honor  of  my 
experiment  belongs  to  the  Gramme  Company. 

In  the  fourth  edition  of  hit  "  Dynamo-Electric 
Machinery/'*  S.  P.  Thompson,  on  page  548,  speaks 
thus  of  the  actions  existing  between  a  magnet  and  a 


348  ELECTRICAL  MEASUREMENTS. 

conductor  through  which  a  current  of  electricity  is 
flowing: 

In  the  first  chapter,  the  definition  was  laid  down  that 
dynamo-electric  machinery  meant  "  machinery  for.  con- 
verting energy  in  the  form  of  mechanical  power  into 
energy  in  the  form  of  electric  currents,  or  vice  versa."  Up 
to  the  present  point  we  have  treated  the  dynamo  solely  in 
its  functions  as  a  generator  of  electric  currents.  We  now 
come  to  the  converse  function  of  the  dynamo,  namely, 
that  of  converting  the  energy  of  electric  currents  into  the 
energy  of  mechanical  motion. 

An  electric  motor,  or,  as  it  was  formerly  called,  an  elec- 
tro-magnetic engine,  is  one  that  does  mechanical  work  at 
the  expense  of  electric  energy  ;  and  this  is  true,  no  matter 
whether  the  magnets  which  form  the  fixed  part  of  the 
machine  be  permanent  magnets  of  steel  or  electro-magnets. 
In  fact,  any  kind  of  dynamo,  independently  excited  or 
self-exciting,  can  be  used  c  mversely  as  a  motor,  though, 
as  we  shall  see,  some  more  appropriately  than  others.  But, 
whether  the  field  magnets  be  of  permanently  magnetized 
steel,  or  of  temporarily  magnetized  iron,  all  these  motors 
are  electro-magnetic  in  principle ;  that  is  to  say,  there  is 
some  part  either  fixed  or  moving  which  is  an  electro- 
magnet, and  which  as  such  attracts  an  I  is  attracted  mag- 
netically. 

*  "  Dynamo  Electric  Machinery  :  A  Manual  for  Students  of  Elec- 
trotechnics,"  by  S.  P.  Thompson,  D.  Sc.,  B.  A.  Fourth  Edition. 
Enlarged  and  Revised.  London:  E.  &  F.  N.  Spon.  1892.  864  pages, 
498  illustrations,  29  plates.  Price.  $9. 


XVII.  —  ELECTRIC     TRANSMISSION     OF 
POWER. 


Any  system  for  the  electric  transmission  of  power 
consists  essentially  of  the  following  parts  : 

(1.)  Of  a  line  conductor  or  circuit  established  be- 
tween the  two  stations. 

(2.)  Of  an  electric  source,  or  battery  of  electric 
sources,  at  one  of  the  stations,  generally  in  the  form 
of  a  dynamo-electric  machine,  or  battery  of  dynamo- 
electric  machines,  for  the  purpose  of  converting 
mechanical  energy  into  electrical  energy. 

(3.)  Of  various  electro- receptive  devices  placed  in 
the  circuit  of  the  line  wire  or  conductor,  in  the  form 
of  electric  motors,  for  the  purpose  of  reconverting 
electrical  energy  into  mechanical  energy. 

The  electro-receptive  devices  may  be  connected  to 
the  line  wire  or  conductor  either  in  series  or  in  mul- 
tiple ;  or,  in  other  words,  the  circuits  established 
between  the  two  stations  may  be  either  constant-cur- 
rent circuits  or  constant-potential  circuits. 

Strictly  speaking,  all  electric  circuits  are  estab- 
lished for  the  transmission  of  electric  energy ;  for, 
(349) 


350  ELECTRICAL  MEASUREMENTS. 

in  all  circuits,  some  form  -of  energy  is  expended  at 
the  source  for  the  purpose  of  producing  electric  en- 
ergy, which  is  transmitted  over  a  line  wire  or  con- 
ductor connecting  such  source  with  an  electro-recep- 
tive device  or  devices,  in  which  such  energy  is  util- 
ized. 

A  system  for  the  electric  transmission  of  power 
differs  from  the  above  merely  in  the  fact  that  in 
such  a  system  mechanical  power  is  transmitted 
through  considerable  distances  between  the  place 
where  it  is  generated  and  the  place  where  it  is  util- 
ized. Some  prime  mover,  as,  for  example,  a  steam 
engine  converts  the  energy  of  heat  into  me- 
chanical energy,  and  which  mechanical  energy  is  con- 
verted by  means  of  a  dynamo-electric  machine  into 
electrical  energy,  is  employed  at  one  end  of  the  line, 
and  the  electrical  energy  so  obtained  is  converted  by 
means  of  such  electro-receptive  devices  as  electric 
motors  into  mechanical  energy  at  the  other  end  of 
the  line  ;  or,  as  is  very  frequently  the  case,  the  me- 
chanical energy  of  a  water  power  is  converted  at  one 
end  of  the  line  by  means  of  a  dynamo  into  electric 
energy,  which  is  employed  as  before. 

The  electric  transmission  of  power  possesses 
marked  advantages  over  any  other  known  system 
for  the  transmission  of  power,  such,  for  example, 


ELECTRIC  TRANSMISSION  OF  POWER.          351 

as  by  means  of  belting,  wire  ropes,  gears,  or  by 
means  of  compressed  air  or  other  fluids. 

Among  the  advantages  possessed  by  the  electric 
transmission  of-  power  the  following  may  be  men- 
tioned : 

(1.)  The  distance  through  which  power  may  be 
economically  transmitted  electrically  is  very  much 
greater  than  by  any  other  known  means.  In  the 
case  of  belting  or  wire  rope  transmission  the  limits 
of  such  transmission  are  measured  in  feet,  while 
in  the  case  of  electric  transmission  they  are  meas- 
ured in  miles.  Hydraulic  transmission,  though 
very  economical  within  certain  limits  of  distance, 
cannot  compete  for  great  distances  with  electric. 

(2.)  Sources  of  power  can  be  utilized  by  systems 
of  electric  transmission  that  would  otherwise  be  im- 
practicable. Suppose,  for  example,  a  waterfall  is 
situated  at  a  fairly  considerable  distance  from  the  city 
or  other  location  where  it  is  desired  to  establish  a 
manufacturing  plant.  A  water-wheel  can  ba  placed 
at  such  waterfall  for  the  purpose  of  converting 
mechanical  energy  by  means  of  a  dynamo  into  elec- 
tric energy,  and  the  electric  energy  so  developed  can 
be  led  by  means  of  conducting  wires  to  the  distant 
place,  where,  on  being  passed  through  electric  motors, 
it  can  be  converted  into  mechanical  power. 


352  ELECTRICAL  MEASUREMENTS. 

(3.)  The  means  by  which  the  driving  power  is 
connected  to  the  distant  driven  mechanism  is  far 
simpler  than  by  any  other  means,  a  mere  pair  of 
wires  or  conductors,  which  may  pass  in  any  direction, 
and  around  any  number  of  corners  or  bends,  being 
all  that  is  necessary  for  this  purpose. 

Contrast  this  simplicity  of  detail  with  belts  or  wire 
ropes,  or  even  with  compressed  air  or  with  water,  and 
the  advantages  will  be  self-evident ;  the  difficulties  of 
leakage  at  joints  and  the  cost  of  construction  in  the 
case  of  the  transmission  by  compressed  air  being 
very  much  greater  than  in  the  case  of  electric  trans- 
mission. 

The  utilization  of  water  power  for  the  production 
of  electric  power,  and  the  transmission  of  such 
power  to  great  distances,  is  rapidly  coming  into  ex- 
tended use.  Numerous  cases  exist  in  which  such 
transmission  is  actually  being  carried  on  over  very 
considerable  distances.  Even  such  rivers  as  Niagara 
near  the  Falls  are  about  to  be  made  to  expend  a 
portion  of  their  energy  in  performing  useful  work, 
and  there  is  every  prospect,  in  the  near  future,  that 
the  number  of  such  practical  applications  will  in- 
crease. 

The  percentage  of  the  relative  efficiencies  of  the 
steam  engine  and  the  electric  motor  is  very  much  in 


ELECTRIC  TRANSMISSION  OF  POWER.          353 

favor  of  the  electric  motor.  The  efficiency  of  the 
best  steam  engine  is  only  in  the  neighborhood  of  17 
per  cent.,  while  that  of  the  electric  motor  can  be 
made  almost  as  high  as  required,  it  often  exceed- 
ing 95  per  cent.  The  question  then  arises,  Can 
we  ever  expect  the  electric  motor  to  replace  the 
steam  engine?  The  answer  would  appear  to  be  that 
such  displacement  must  certainly  occur  whenever 
electricity  can  be  produced  more  economically  by 
the  burning  of  coal,  or  by  means  other  than  through 
the  intervention  of  the  steam  engine. 

As  long  as  the  best  method  of  producing  electrical 
energy  is  limited  to  driving  a  dynamo  by  a  steam 
engine  the  steam  engine  must  of  course  hold  its  own. 
If,  however,  the  discovery  is  ever  made — and  such 
discovery  is  by  no  means  improbable — of  a  means  of 
economically  producing  electricity  directly  from  the 
burning  of  coal,  then  the  steam  engine  will  certainly 
be  replaced  by  the  electric  motor. 

Even  during  the  present  day  the  electric  motor 
can  economically  compete  with  steam  under  the 
following  circumstances: 

(1.)  In  certain  cases  where  it  replaces  horse 
or  other  animal  power. 

The  cost  of  producing  and  distributing  electric 
energy  for  replacing  the  power  of  horses  on  street 


354  ELECTRICAL  MEASUREMENTS. 

railways  is  much  less  than  feeding  and  caring  for 
the  horses. 

(2.)  For  all  cases  where  an  available  water  power 
exists,  even  though  at  considerable  distances  from 
the  places  where  its  energy  is  to  be  utilized. 

The  energy  of  moving  water,  except  in  the  case  of 
very  irregular  streams,  can  generally  be  caused  to 
produce  mechanical  power  at  a  smaller  expense  than 
by  the  steam  engine. 

(3.)  In  places  where  only  small  amounts  of  power 
are  required. 

A  saving  can  generally  be  made,  when  but  a  com- 
paratively small  power  is  needed,  by  using  electric 
power  under  circumstances  where  a  steam  engine  and 
boiler  and  the  services  of  an  engineer  can  thereby  be 
dispensed  with.  In  manufacturing  centres  where 
rentals  are  high  the  mere  saving  of  the  space  re- 
quired by  a  steam  plant  secures  so  great  economy 
as  to  permit  the  displacement  of  steam  power  by 
electric  power.  This  is  now  done  economically  in  a 
number  of  cases  either  by  the  establishing  of  a  large 
central  power  station  driven  by  water,  or  by  steam 
power,  and  supplying  outlying  districts  with  electric 
power. 

The  various  systems  of  telegraphic  and  telephonic 
communication  afford  excellent  illustrations  of  the 


ELECTRIC  TRANSMISSION  OF  POWER.          355 

actual  transmission  of  electric  power.  In  the  tele- 
phone the  voice  of  the  speaker,  acting  as  the  driving 
power,  converts  mechanical  energy  into  electrical 
energy,  which  is  transmitted  over  a  line  wire  or  con- 
ductor, and,  passing  at  the  distant  end  through  a 
form  of  electro-receptive  device  called  a  receiver, 
converts  the  electrical  energy  into  mechanical  energy, 
which  in  its  turn  reproduces  the  articulate  speech 
spoken  into  the  transmitting  instrument. 


FIG.  163.— TELPHERAGE  SYSTEM. 

The  electric  transmission  of  power  is  not  limited 
to  the  case  where  electric  energy  is  sent  into  a  line 
wire  or  conductor  at  one  end,  and  is  utilized  at  the 
other  end  only  ;  for,  in  many  cases,  the  energy  is 
taken  from  such  line  wire  or  conductor  at  inter- 
mediate points  as  well  as  at  its  further  end. 

Examples  of  this  are  seen  in   various  telpherage 


356  ELECTRICAL  MEASUREMENTS. 

systems,  in  the  porte-electric  system,  and,  especially, 
in  any  of  the  well-known  systems  of  electric  railways. 

The  telpherage  system,  an  invention  of  Fleem- 
ing  Jenkin,  is  a  system  whereby  carriages  sus- 
pended from  electric  conductors  are  propelled  or 
driven  along  said  conductors  by  the  action  of  electric 
motors  that  take  the  current  required  to  energize 
them  from  the  conductors  over  which  they  move. 

The  telpherage  system  shown  in  Fig.  163  is  called 
the  cross-over  or  parallel  system.  In  this  system  two 
conductors,  connected  to  a  dynamo-electric  machine, 


FIG.  164.—  CIRCUIT  FOR  TELPHERAGE  SYSTEM. 

which  maintains  them  at  a  constant  difference  of 
potential,  are  caused  to  cross  each  other  at  regular 
intervals  in  the  manner  shown  in  Fig.  164.  By  this 
means  the  wires  or  conductors  on  each  side  of  the  road 
are  alternately  positive  and  negative,  and  two  lines 
are  thus  provided  —  an  up  and  a  down  line. 

The  train  of  cars  as  shown  at  L  T,  or  at  L'  T,  is 
of  sufficient  length  to  make  contact  with  two  ad- 
joining sections  at  the  same  time.  The  wheels  of 
each  train  are  insulated  from  their  trucks,  but  are 
connected  together  in  pairs  by  means  of  a  con- 


ELECTRIC  TRANSMISSION  OF  POWER.          357 

ductor.  Therefore,  as  the  train  passes  over  the 
track  a  current  flows  through  the  motor  on  each 
train  from  the  positive  to  the  negative  section. 

Various  other  forms  have  been  proposed  for  tel- 
pherage systems. 

The  porte-electric  system  is  the  name  given  to  a 
system  of  electric  carriage  or  transportation,  by 
means  of  the  successive  attractions  which  a  number 
of  hollow  helices  of  insulated  wire  exert  on  a  hollow 
cylindrical  core  of  iron. 

The  cylindrical  car  forms  the  movable  core  of  a 
number  of  helical  coils.  As  it  moves  through  the 
helices  it  closes  the  circuit  of  an  electric  current 
through  the  coils  which  lie  in  advance  of  it,  and 
opens  the  circuit  of  those  coils  through  which  it  has 
just  passed. 

In  this  manner  the  solenoidal  core  advances  in  a 
line  coincident  with  the  axis  of  the  coils,  being 
virtually  sucked  through  them  by  their  magnetic 
attraction. 

In  the  porte-electric  system  of  transportation  the 
electric  motor  becomes  practically  a  mere  mass  of 
iron,  as  shown  in  Fig.  165.  The  system  is  applica- 
ble to  the  carriage  of  maite  or  other  comparatively 
light  articles  at  high  speed. 

The  solenoidal  coils  are  shown  in  Fig.  166,   a 


358  ELECTRICAL  MEASUREMENTS. 

section  of  the  track  of  the  porte-electric  system  as 
operated  on  an  experimental  plant  at  Dorchester, 
Mass.  The  length  of  this  track  was  2,784  feet,  the 
solenoidal  core  or  car  weighed  about  500  pounds, 
and  could  carry  about  10,000  letters.  It  was  pro- 
vided with  two  flanged  wheels  placed  above  and 
below. 

The  solenoidal  coils,  whose  attractive  power  caused 
the  motion  of  the  car,  embraced  the  track  and  the 
movable  core  or  carrier.  Each  coil  was  formed  of 


FlG.  165.— PORTE-KLKCTRIC  CAR. 

630  turns  of  number  14  copper  wire.  A  speed  of 
about  34  miles  an  hour  was  reached. 

By  far  the  most  successful  system  of  electric  pro- 
pulsion of  movable  carriages  is  seen  in  the  various 
systems  of  electric  railroads  which  are  now  in  such 
extended  successful  use.  In  such  systems  cars  are 
propelled  by  means  of  electric  motors. 

The  current  that  drives  the  motor  is  derived  either 
from  storage  batteries  placed  on  the  cars,  or  from  a 


ELECTRIC  TRANSMISSION  OF  POWER.  359 

dynamo-electric  machine  especially  designed  for  this 
purpose  and  situated  at  some  point  on  the  road. 

The  current  from  the  generating  dynamo  in  this 
latter  case  is  led  to  the  oar  along  the  route  by  means 
of  suitable  conductors,  and  is  taken  from  such  con- 
ductors and  passed  into  the  motors. 

Systems  for  electric  railroads  may,  therefore,  be 
divided  into: 


FIG.  166. -PORTE-ELECTRIC  TRACK. 

(1.)  The  Independent  System,  where  the  driving 
current  is  derived  from  primary  or  secondary  bat- 
teries placed  on  the  car  ;  and, 

(2.)  The  Dependent  System,  where  the  driving 
current  is  taken  from  conductors  placed  somewhere 
outside  the  car  by  means  of  sliding  or  rolling  con- 
tacts. 

The  dependent  system  of  motive  power  for  electric 
railroads  includes  three  distinct  varieties. 


360  ELECTRICAL  MEASUREMENTS. 

(1.)  The  Underground  System. 

(2.)  The  Surface  System. 

(3.)  The  Overhead  System. 

In  these  systems,  since  the  current  is  led  from 
the  generating  dynamo  through  line  wires  or  con- 
ductors that  supply  the  current  to  the  motors  by 
means  of  rolling  or  sliding  contacts,  such  wires  or 
conductors  are  necessarily  bare  or  uninsulated. 
They  must,  therefore,  be  suitably  supported  on  in- 
sulators in  order  to  prevent  leakage,  and  such  insu- 
lators must  possess  comparatively  high,  insulating 
powers. 

In  order  to  avoid  the  difficulties  arising  from  bare 
underground  wires,  systems  have  been  devised  in 
which  the  conductors  are  entirely  surrounded  by  in- 
sulating materials,  and,  either  actual  temporary  con- 
tacts are  made  as  the  car  passes,  or  the  car  takes  the 
energy  required  to  propel  it  by  means  of  induction 
from  the  underground  conductors.  None  of  these 
systems,  however,  have  come  into  any  extended 
use. 

In  the  underground  system  a  continuous  bare  con- 
ductor, placed  in  an  open-slotted  conduit,  supplies 
the  driving  current  by  means  of  traveling  conduc- 
tors or  trailers  placed  on  the  car  and  connected  with 
the  electric  motors  by  rolling  or  sliding  over  it. 


ELECTRIC  TRANSMISSION  OF  POWER,  361 

In  the  surface  system  the  wires  or  conductors 
connected  to  the  generating  dynamo,  instead  of 
being  placed  underground  in  an  open-slotted  con- 
duit, are  placed  directly  on  the  surface  of  the  street 
or  road-bed  and  the  current  taken  from  them  by 
suitable  contacts  placed  on  the  car. 

The  overhead  system  is  the  one  that  has  come 
into  the  most  extended  use.  In  actual  practice  it 
operates  by  means  of  a  continuous  bare  conductor 
suspended  by  suitable  supports  over  the  roadway 
or  bed. 

The  current  required  to  drive  the  car  is  taken 
from  the  overhead  wire  or  conductor  by  means  of 
a  traveling  wheel  or  roller  called  a  trolley.  In 
these  systems  the  overhead  wire  is  either  arranged 
in  a  continuous  metallic  circuit  or  the  ground  is 
used  for  the  return  circuit.  The  latter  plan  is  in 
most  general  use. 

The  electric  motor  is  placed  underneath  the  car 
on  what  is  called  a  motor  truck,  and  is  geared 
to  the  axle  of  the  car. 

Most  electric  motors  have  their  greatest  efficiency 
when  run  at  high  speeds,  since  then  their  counter- 
electromotive  force  is  greatest.  In  order  to  reduce 
the  speed  of  the  car  to  the  limit  of  safety  required 
for  use  in  crowded  cities,  and  yet  to  permit  the 


362  ELECTRICAL  MEASUREMENTS. 

motor  to  run  at  a  high  speed,  some  form  of  reduc- 
tion gear  is  employed. 

Improvements  have  recently  been  made  in  what 
are  known  as  low-speed  motors,  by  which  fairly  low 
speeds  can  be  obtained,  without  any  reduction  gear 
whatever,  with  a  fair  degree  of  efficiency. 

In  order  to  regulate  the  speed  of  the  motor  vari- 
ous devices  are  employed,  the  object  of  which  is  to 
vary  the  current  in  the  motor  circuit.  These  devices 
consist  essentially  of  rheostats,  or  resistances,  which 
are  introduced  into  or  removed  from  the  motor  cir- 


FIG.  167.— LIVE  TKOLLKY  CROSSING. 

cuit  by  the  movement  of  a  lever,  or  the  movement 
of  a  wheel,  which  forms  part  of  the  circuit  and  moves 
over  contact  plates  connected  with  the  various  resist- 
ance coils,  and  also  of  devices  for  readily  effecting 
different  couplings  of  the  field  coils,  etc.  Like  all 
such  devices,  the  portions  handled  are  carefully  in- 
sulated from  the  circuit. 

In  order  to  change  the  direction  of  rotation  of  the 
motor,  and  thus  reverse  the  direction  in  which  the 
car  moves,  various  devices  are  employed,  which  de- 
pend either  on  the  changing  of  the  field,  or  reversing 


ELECTRIC  TRANSMISSION  OF  POWER.          363 

the  direction  of  the  current  in  the  field  or  in  the 
armature. 

In  order  to  protect  the  electrical  apparatus  from 
an  accidental  discharge  of  lightning  through  the 
bare  conductors,  some  form  of  lightning  arrester  is 
connected  with  the  line. 

Various  forms  are  given  to  the  trolley  arms  or 
poles.  A  well-known  form  is  known  as  the  drop- 
trolley.  In  this  form  the  movement  of  a  lever  con- 
nected with  the  trolley  pole  causes  the  trolley  wheel 
to  drop  away  from  the  line  wire.  A  motion  in  the 


FIG.  168.-  TROLLEY  CROSS-OVER. 

opposite  direction  raises  the  trolley  wheel  upward  to 
the  proper  elastic  pressure. 

In  systems  where  the  overhead  line  or  conductor 
consists  of  two  wires  forming  a  continuous  metallic 
circuit  a  double  trolley  wheel  must  be  employed, 
one  wheel  to  carry  the  current  to  the  motor  and  the 
other  to  return  it  after  it  has  passed  through  the 
motor. 

A  trolley  cross-over  is  a  device  by  means  of  which 
a  trolley  is  enabled  to  pass  without  interruption  over 
points  where  different  lines  cross  one  another.  A 
trolley  cross-over  is  shown  in  Fig.  168. 


364 


ELECTRICAL  MEASUREMENTS. 


The  position  of  the  trolley  arm,  etc.,  in  the  case 
of  a  form  of  double  deck  car  designed  by  the  Pull- 
man Company  for  street  railway  services,  is  shown 


FIG.  169.— PULLMAN  STREET  CAR. 

in  Fig.  169  As  will  be  seen  from  an  inspection  of 
the  figure,  a  spiral  stair  case  is  provided  at  either 
side  of  the  car,  near  the  centre,  to  ensure  ready 
communication  with  the  two  compartments. 


ELECTRIC  TRANSMISSION  OF  POWER.          365 

EXTRACTS  FROM  STANDARD  WORKS. 
Crosby  and   Bell,  in    "The  Electric  Railway   in 
Theory  and  Practice/'  *  speaking  of  the  efficiency  of 
electric  traction,  on  page  202  say  : 

Whatever  may  be  the  advantages  of  electric  traction, 
whatever  its  convenience  as  a  means  of  rapid  transit,  it  is 
on  its  efficiency  that  its  ultimate  importance  must  depend. 
We  must  realize  at  the  start  that  the  electric  motor  is  not 
a  prime  mover,  a  fundamental  source  of  energy ;  it  only 
furnishes  a  very  perfect  and  elegant  means  of  utilizing 
electrical  energy,  already  generated  by  some  prime  mover, 
at  the  point  where  it  may  be  most  convenient  to  employ  it, 
whether  that  point  be  fixed,  as  in  the  case  of  stationary 
motors,  or  moving,  as  in  tne  case  of  street  railways.  Ad- 
vantages may  be  and  are  gained  by  employing  motors 
sufficient  to  offset  considerable  losses  in  the  necessary 
transmission  and  tianstormaiion  of  electrical  energy,  but  if 
these  losses  rise  above  a  certain  amount  the  system  must 
inevitably  be  a  commercial  failure. 

Let  us  look,  then,  deliberately  at  the  series  of  transmis- 
sions and  transformations  necessary  in  electric  traction,  and 
form  as  close  estimates  as  possible  of  the  losses,  their  mag- 
nitudes, and  the  most  practicable  means  for  reducing  them  to 
a  more  satisfactory  figure.  The  first  transformation  of 
energy  is  from  the  pressure  of  steam  generated  in  the 


*"The  Electric  Railway  in  Theory  and  Practice,"  by  Oscar  T. 
Crosby  and  Louis  Bell,  Ph.  D.  Second  edition,  revised  and  en- 
larged. New  York:  The  W.  J.  Johnston  Company,  Limited,  1893. 
412  pages,  182  illustrations.  Price,  $2.50. 


366  ELECTRICAL  MEASUREMENTS. 

boilers  to  the  rotary  motion  produced  by  the  engine  and 
employed  in  driving  dynamos. 

Then  the  mechanical  energy  obtained  is  first  trans- 
ferred, through  the  medium  of  shafting  or  belting,  to  the 
dynamo,  where  it  is  again  transformed  and  appears  as  elec- 
trical current  on  the  line.  In  this  convenient  shape  it  is 
transferred,  with  little  loss,  to  the  point  on  the  line  where 
the  motor  or  motors  may  happen  to  be.  There  it  undergoes 
another  transformation  in  the  motor  back  to  mechanical 
energy,  which  is  then  transferred,  through  the  medium  of 
gearing  of  one  sort  or  another,  from  the  armature  shaft  to 
the  car  wheels. 

Fortunately,  the  losses  at  several  points  in  this  somewhat 
complicated  system  of  transmutations  are  comparatively 
small ;  and  for  practical  purposes  we  may  consider  the 
losses  to  be  substantially  as  follows  :  first,  the  losses  in  the 
engine  and  attachments  ;  second,  those  in  the  dynamo  ; 
third,  those  on  the  line  ;  fourth,  those  in  the  motor;  fifth, 
those  in  the  gearing.  Luckily,  not  all  these  are  serious. 
In  the  art  of  electric  traction  as  to-day  practiced  their  rela- 
tive magnitudes  are  about  as  follows  :  the  most  formidable 
are  the  first  and  last ;  they  are  of  about  the  same  magni- 
tude, varying  enough  in  different  cases  to  render  it  quite 
impossible  to  say  off  hand  which  is  the  larger.  Then  come 
the  losses  in  the  dynamo  and  motor,  generally  smaller  than 
either  of  the  former,  and  that  in  the  motor  being  some- 
what the  larger.  Finally,  the  loss  on  the  line,  which  in 
many  cases  is  the  least  of  all.  Reduction  of  gearing  has  in 
some  cases  made  that  source  of  loss  relatively  small,  but  too 
often  at  the  expense  of  motor  efficiency. 


XVIIL— PRIMER  OF  PRIMERS. 


Various  methods  are  adopted  for  measuring  the 
strength  of  currents  that  pass  in  any  circuit.  The 
most  important  of  these  are  as  follows  : 

(1.)  The  voltametric  method,  based  on  the  elec- 
trolytic power  of  the  current. 

(2.)  The  calorimetric  method,  based  on  the  heat- 
ing power  of  the  current. 

(3.)  The  galvanometric  method,  based  on  the 
magnetic  power  of  the  current. 

(4.)  The  indirect  method,  in  which  the  electro- 
motive force  and  the  resistance  are  first  measured, 
and  the  current  strength  then  calculated. 

In  the  voltametric  method  the  current  strength 
passing  is  measured  by  means  of  the  amount  of 
chemical  decomposition  it  effects  in  a  liquid  placed 
in  an  instrument  called  a  voltameter. 

Voltameters  may  be  divided  into  two  classes ; 
namely,  volume  voltameters  and  weight  voltame- 
ters. 

In  the  calorimetric  method  the  current  strength 
passing  is  measured  by  means  of  the  increase  in  tem- 
perature it  produces  in  a  given  time  in  a  known 
(367) 


368  ELECTRICAL  MEASUREMENTS. 

weight  of  liquid  placed  in  an  instrument  called  a 
calorimeter.  The  heat  produced  by  the  passage 
of  the  electrical  current  through  the  conductor  is 
proportional  to  the  resistance  of  the  conductor,  to 
the  square  of  the  current  passing,  and  to  the  time 
the  current  continues  to  pass. 

In  the  galvanometric  method  the  current  strength 
passing  is  measured  by  means  of  the  amount  of  de- 
flection it  produces  in  a  magnetic  needle  placed  in 
the  field  of  the  circuit. 

In  a  galvanometer  the  direction  in  which  the  needle 
is  deflected  depends  on  the  direction  in  which  the 
the  current  flows  through  the  deflecting  circuit  as 
well  as  on  the  position  of  the  needle  as  regards  such 
circuit ;  namely,  whether  above  or  below,  to  the 
right  or  to  the  left  of  such  circuit. 

In  all  cases  the  galvanometer  needle  tends  to  come 
to  rest,  under  the  deflecting  power  of  the  current,  in 
a  position  at  right  angles  to  the  direction  in  which 
the  current  is  passing. 

Various  forms  maybe  given  to  galvanometers; 
among  the  most  important  of  these  are  the  sine  gal- 
vanometer, the  tangent  galvanometer,  the  ballistic 
galvanometer,  the  torsion  galvanometer,  the  astat- 
ic galvanometer,  the  mirror  or  reflecting  galvan- 
ometer and  the  differential  galvanometer. 


PRIMER  OF  PRIMERS.  369 

In  the  commercial  distribution  of  electricity  vari- 
ous devices  called  electric  meters  are  employed  for 
measuring  and  recording  the  quantity  of  electricity 
that  passes  in  a  given  time  through  any  consump- 
tion circuit. 

Electric  meters,  though  of  a  great  variety  of 
forms,  can  be  grouped  under  the  following  general 
classes;  namely: 

(1.)  Electro-magnetic  meters,  in  which  the  cur- 
rent passing  is  measured  by  its  magnetic  effects. 

(2.)  Electro-chemical  meters,  in  which  the  cur- 
rent passing  is  measured  by  the  electrolytic  decom- 
position it  produces. 

(3.)  Electro-thermal  meters,  in  which  the  current 
passing  is  measured  by  movements  produced  by  the 
increase  in  temperature  of  a  resistance  through 
which  the  current  passes,  or  by  means  of  a  difference 
of  weight  produced  by  the  evaporation  of  a  liquid  by 
means  of  the  heat  generated  by  the  current. 

(4.)  Electric  time  meters,  in  which  no  attempt  is 
made  to  measure  the  current  passing,  but  in  which 
a  record  is  kept  of  the  number  of  hours  during 
which  the  current  flows  through  the  consumption 
circuit. 

The  electromotive  force  of  a  source,  or  the  dif- 
ference of  potential  between  any  two  points  of  a  cir- 


370  ELECTRICAL  MEASUREMENTS. 

cuit,  can  be  measured  in  a  variety  of  ways,  among 
the  most  important  of  which  are  : 

(1. )  By  the  use  of  galvanometers,  or  galvanometer- 
voltmeters. 

(2.)  By  the  use  of  electrometers,  or  electrometer- 
voltmeters. 

(3.)  By  the  method  of  weighing,  or  by  balance- 
voltmeters. 

(4.)  By  the  indirect  method  in  which  the  current 
strength  and  the  resistance  are  first  determined  and 
the  electromotive  force  then  calculated  from  the 
formula  E  -  C  R. 

In  galvanometer-voltmeters  the  difference  of  po- 
tential is  determined  by  the  amount  of  the  deflection 
of  a  magnetic  needle  produced  by  a  current  which 
flows  through  a  coil  of  insulated  wire,  and  which 
current  results  from  the  difference  of  potential  exist- 
ing between  two  points  of  the  circuit  whose  difference 
of  potential  is  to  be  measured.  The  resistance  of  the 
instrument  remaining  constant  the  value  of  such 
current  depends  entirely  on  the  difference  of  potential 
of  the  points  to  which  the  voltmeter  is  connected. 

The  deflection  of  the  needle  may  be  made  against 
the  earth's  field,  against  the  field  of  a  permanent 
magnet,  against  the  action  of  a  spring,  or  against  the 
force  of  gravity  acting  on  a  weight. 


PRIMER  OF  PRIMERS.  371 

In  another  form  of  voltmeter  the  strength  of  the 
current  passing,  and  hence  the  difference  of  poten- 
tial producing  it,  is  determined  by  means  of  the 
heat  it  generates. 

In  the  quadrant  electrometer  the  difference  of 
potential  of  a  circuit  is  determined  by  the  electro- 
static attractions  and  repulsions  of  an  easily  moved 
metallic  needle  suspended  between  insulated  metallic 
quadrants. 

In  the  capillary  electrometer  the  difference  of  po- 
tential is  determined  by  the  movements  of  a  drop  of 
acid  in  a  capillary  tube  filled  with  mercury. 

Standard  voltaic  cells  are  convenient  devices  for 
obtaining  a  known  difference  of  potential  which  is 
employed  to  determine  an  unknown  difference  of 
potential  by  balancing  or  opposing  it. 

Some  of  the  most  frequently  employed  standard 
voltaic  cells  are  Clark's  standard  cell,  Ealeigh's 
modification  of  Clark's  standard  cell,  Fleming's 
standard  cell  and  Lodge's  standard  cell. 

In  these  cells  the  electromotive  force  is  constant 
only  when  certain  conditions  are  rigorously  main- 
tained. 

The  resistance  of  a  circuit  or  part  of  a  circuit  can 
be  determined  in  a  great  variety  of  ways.  Among 
the  most  important  of  these  are: 


372  ELECTRICAL  MEASUREMENTS, 

(1.)  The  method  of  substitution. 

(2.)  The  comparison  of  the  deflections  of  galvan- 
ometer needles. 

(3.)  The  use  of  differential  galvanometers. f 

(4.)  The  use  of  a  Wheatstone  bridge  in  connec- 
tion with  a  box  of  resistance  coils. 

(5.)  The  indirect  method  ;  that  is,  by  the  formula 

E 

R  =  —. 

C 

In  measuring  the  resistance  of  a  circuit  by  means 
of  a  Wheatstone  bridge  the  circuit  is  caused  to 
branch  and  flow  through  four  arms,  two  of  which 
are  placed  in  each  branch  of  the  circuit.  A  galvan- 
ometer is  made  to  join  or  bridge  parts  of  the 
branched  circuit  lying  between  the  resistances  placed 
in  each  branch.  If  the  resistance  in  one  of  these 
arms  and  the  relative  resistance  in  two  of  the  remain- 
ing arms  are  known,  the  resistance  of  the  fourth 
arm  can  be  determined  from  the  value  which  the 
remaining  resistance  will  have  when  no  current 
flows  through  the  galvanometer. 

"Wire  gauges  are  means  for  determining  accurately 
the  diameter  of  wires  or  other  conductors. 

In  1786  Galvani  made  an  observation  concerning 
the  convulsive  movements  of  the  legs  of  a  recently 


PRIMER  OF  PRIMERS.  373 

killed  frog.  A  few  years  later  this  observation  of 
Galvuhi's  led  Volta  to  the  invention  of  the  voltaic 
pile. 

A  voltaic  cell  generally  consists  of  two  dissim- 
ilar metals,  called  a  voltaic  couple,  dipping  into  a 
liquid  called  an  electrolyte.  A  difference  of  poten- 
tial is  generated  by  the  contact  of  the  dissimilar 
metals  through  the  agency  of  the  electrolyte,  but  the 
energy  required  to  maintain  a  continuous  flow  arises 
from  the  chemical  potential  energy  liberated  by  means 
of  the  solution  of  one  of  the  metals  by  the  electrolyte. 

In  a  voltaic  cell  one  of  the  metals  of  the  voltaic 
couple  is  dissolved  or  acted  upon  chemically  by  the 
electrolyte  ;  the  other  is  not  acted  on.  The  former 
is  generally  called  the  positive  plate  and  the  latter 
the  negative  plate. 

In  a  voltaic  cell  the  polarity  is  as  follows  :  the 
negative  terminal  of  the  battery  is  the  terminal  that 
is  connected  to  the  plate  that  is  dissolved  or  acted 
on  by  the  electrolyte  ;  the  positive  terminal  is  the 
terminal  that  is  connected  to  the  other  plate. 

By  a  convention  it  is  agreed  to  call  that  pole  of  an 
electric  source,  out  from  which  the  current  flows,  the 
positive  pole  of  the  source,  and  that  pole  into  which 
the  current  flows,  the  negative  pole  of  the  source. 
Inasmuch  as  within  the  cell,  beneath  the  liquid,  the 


374  ELECTRICAL  MEASUREMENTS. 

current  flows  in  the  opposite  direction,  it  is  assumed 
for  convenience  that  the  metal  most  acted  on  is  posi- 
tive, and  the  metal  least  acted  on  is  negative.  This, 
as  will  be  seen,  makes  the  polarity  of  a  voltaic  cell, 
within  the  liquid,  opposite  to  that  outside  the  liquid. 

During  the  action  of  a  voltaic  cell  there  is  a  ten- 
dency for  the  hydrogen  to  collect  on  the  surface  .of 
the  negative  plate.  This  is  called  the  polarization 
of  the  cell. 

The  polarization  of  a  voltaic  cell  tends  to  de- 
crease the  current  that  such  cell  can  furnish — 

(1.)  On  account  of  the  counter-electromotive 
force  which  such  collection  of  gas  produces,  thus 
decreasing  the  effective  electromotive  force  of  the  cell. 

(2.)  On  account  of  the  increased  resistance  of  the 
cell,  due  to  the  bubbles  of  gas  so  collected. 

The  ill  effects  of  polarization  may  be  avoided — 

(1.)  Mechanically;  by  brushing  off  the  bubbles  of 
gas,  or  by  permitting  them  to  readily  escape  from 
the  roughened  surfaces  of  the  plate. 

(2.)  Chemically]  by  surrounding  the  surf  ace  of  the 
negative  plate  by  a  powerful  oxidizing  substance. 

(3.)  Electro-chemically;  by  depositing  on  the  sur- 
face of  the  negative  plate  a  coating  or  layer  of  the 
same  metal  as  that  of  which  the  plate  is  composed; 

Voltaic  cells  may  be  divided  into  two  great  classes; 


PRIMER  OF  PRIMERS.  375 

namely,  single-fluid  cells,  and  double-fluid  cells.  In 
the  former  there  is  a  single  electrolyte,  in  the  latter, 
there  are  two  electrolytes. 

In  the  single-fluid  cell,  as  the  name  indicates,  both 
elements  are  immersed  in  the  same  electrolyte.  In 
the  double-fluid  cell,  each  element  is  immersed  in  a 
separate  electrolyte,  the  fluids  being  kept  from  mix- 
ing either  by  means  of  a  porous  partition,  or  cell,  or 
by  means  of  their  different  densities. 

The  principal  single-fluid  cells  are  the  bichrom- 
ate, the  Smee  and  the  zinc-copper.  The  principal 
double-fluid  cells  are  the  Dauiell,  the  Grove,  the 
Bunsen,  and  the  Leclanche. 

Of  the  great  variety  of  voltaic  cells  that  have  been 
devised,  two  only  have  survived  in  the  struggle  for 
existence ;  namely,  the  gravity  and  the  Leclanche1. 
The  former  is  called  a  closed-circuited  cell,  because 
it  can  remain  for  an  indefinite  time  on  closed  circuit 
without  polarization.  The  second  is  called  an  open- 
circuited  cell,  because  it  is  only  suitable  for  use  dur- 
ing short  intervals  of  time. 

The  gravity  cell  is  used  principally  in  telegraphic 
circuits  ;  the  Leclanche  for  the  circuits  of  annun- 
ciators, electric  bells,  or  for  similar  purposes. 

In  the  thermo  cell,  invented  by  Seebeck  in  1821, 
two  dissimilar  metals,  formed  into  a  circuit  by  solder- 


376  ELECTRICAL  MEASUREMENTS. 

ing  their  ends  together,  produce  an  electric  current 
when  one  of  their  junctions  is  maintained  at  a  dif- 
ferent temperature  from  the  other. 

Thermo-electric  cells,  like  voltaic  cells,  consist 
of  two  dissimilar  substances  which  form  a  voltaic 
couple. 

A  thermo-electric  battery  consists  of  a  number  of 
thermo-electric  cells  connected  so  as  to  act  as  a 
single  electric  source. 

Thermo-electric  batteries  are  connected  in  series 
in  order  to  add  together  the  weak  electro-motive 
force  produced  by  each  cell. 

A  photo-electric  cell  consists  of  a  sheet  or  extended 
layer  of  selenium,  so  arranged  that  a  difference  of 
potential  is  produced  when  one  of  its  faces  is  dif- 
ferently illumined  from  the  other. 

A  selenium  cell  is  sometimes  called  a  selenium  re- 
sistance, because  its  electric  resistance  undergoes 
marked  variations  when  its  faces  are  exposed  to 
differences  in  intensity  of  illumination. 

The  following  points  are  asserted  by  Van  TJljanin 
concerning  selenium  cells  : 

(1.)  That  the  electromotive  force  produced  by 
exposure  causes  a  current  to  flow  from  the  illumined 
to  the  non-illumined  electrode. 

(2.)  That  such  electromotive  force  immediately 


PRIMER  Of  PRIMERS.  877 

• 

appears  and  disappears  on  "exposure  to  or  removal 
from  the  light. 

(3.)  The  sensitiveness  of  the  cell  decreases  with 
age.  This  change  is  probably  due  to  an  allotropic 
modification  occurring  in  the  selenium. 

(4.)  When  heat  rays  are  absent  the  electromotive 
force  is  proportional  to  the  intensity  of  the  illumina- 
tion. 

A  crystal  of  tourmaline  acts  as  an  electric  source 
and  produces  differences  of  potential  when  its  ends 
or  poles  are  unequally  heated. 

When  a  liquid  is  forced  through  a  capillary 
tube  differences  of  potential  are  produced,  and  the 
tube  and  its  moving  column  act  as  an  electric 
source. 

Currents  produced  by  the  movements  of  liquid 
through  capillary  tubes  are  called  diaphragm  currents. 
The  electromotive  force  of  such  currents  depends 

(1.)  On  the  material  of  the  diaphragm. 

(2.)  On  the  nature  of  the  liquid. 

(3.)  On  the  pressure  required  to  force  the  liquid 
through  the  diaphragm. 

Plants  and  animals  act  as  independent  sources  of 
electricity. 

Any  system  for  the  distribution  of  electric  energy 
embraces  the  following  parts;  namely, 


378  ELECTRICAL  MEASUREMENTS. 

(1.)  Various  electric  sources  or  batteries  of  elec- 
tric sources. 

(2.)  Various  electro- receptive  devices. 

(3.)  Conductors  or  leads  connecting  the  sources 
with  the  electro-receptive  devices. 

Among  the  most  important  systems  for  the  dis- 
tribution of  electric  energy  are  the  following  : 

(1.)  A  system  of  distribution  by  means  of  direct  or 
continuous  currents. 

(2.)  A  system  of  distribution  by  means  of  alternat- 
ing currents. 

(3.)  A  system  of  distribution  by  means  of  storage 
batteries  or  secondary  generators. 

(4.)  A  system  of  distribution  by  means  of  motor 
generators. 

Distribution  by  means  of  direct  or  continuous  cur- 
rents, though  of  a  variety  of  forms,  can  be  arranged 
under  the  following  heads: 

(1.)  A  system  of  constant  current  distribution  in 
which  the  current  is  distributed  over  a  line  wire  or 
conductor  in  such  a  manner  that  its  strength  is 
maintained  approximately  constant,  the  electro- 
motive force  of  the  source  being  changed  with 
changes  in  the  resistance  of  the  circuit. 

(2.)  A  system  of  constant  potential 'distribution 
in  which  a  constant  difference  of  potential  is  main- 


PRIMER  OF  PRIMERS.  379 

tained  on  the  leads  or  conductors  to  which  the  elec- 
tro-receptive devices  are  connected. 

In  a  system  of  constant  current  distribution,  in 
which  the  receptive  devices  are  connected  to  the 
line  in  series,  each  device  added  increases  the  total 
resistance  of  the  circuit,  and,  in  order  to  maintain  a 
constant  current  strength,  the  electromotive  force 
must  be  correspondingly  increased. 

In  the  constant  potential  circuit  each  electro-re- 
ceptive device  added  in  multiple  to  the  mains  or 
leads  decreases  the  total  resistance  of  the  circuit, 
while  each  one  removed  therefrom  increases  the  total 
resistance. 

Various  means  are  devised,  whereby,  in  a  constant 
current  circuit,  variations  in  the  electromotive  force 
of  the  source  are  effected  in  order  to  maintain  the 
current  strength  constant  despite  changes  in  the 
load  on  the  circuit.  These  devices  are  either  auto- 
matic or  non-automatic. 

The  first  considerable  voltaic  arc  taken  between 
carbon  points  was  obtained  by  Davy  in  1809.  A 
carbon  voltaic  arc  consists  of  a  stream  of  highly 
heated  incandescent  carbon  vapor,  which  proceeds 
from  a  cavity  in  the  positive  carbon,  and  is  directed 
toward  the  negative  carbon. 

Voltaic  arcs  may  be  established  between  metallic 


380  ELECTRICAL  MEASUREMENTS. 

electrodes.  Such  arcs  are  generally  less  luminous 
than  carbon  arcs,  but  are  longer  and  possess  a 
color  characteristic  of  the  volatilized  metal. 

During  the  formation  of  a  carbon  arc  the  carbon 
electrodes  are  consumed — the  positive  more  rapidly 
than  the  negative.  When  used  in  an  arc  lamp  some 
device  must  be  employed  to  maintain  them  a  con- 
stant distance  apart.  This  is  generally  effected  by 
placing  the  two  carbons  one  above  the  other,  and 
causing  the  upper  or  positive  carbon  to  drop  toward 
the  lower  or  negative  carbon  at  intervals  dependent 
on  the  distance  between  the  carbons. 

Arc  lamps  are  generally  placed  in  the  distribution 
circuit  in  series.  In  such  cases  each  lamp  is  pro- 
vided with  au  automatic  cut-one  which  removes  it 
from  the  circuit  when  it  fails  to  properly  operate, 
and,  at  the  same  time,  establishes  a  by-path  or  short 
circ-uit  past  the  lamp  so  as  not  to  interfere  with  the 
working  of  the  remainder  of  the  circuit. 

An  arc  lamp  is  generally  provided  with  a  hood  of 
a  conical  form,  which  serves  the  double  purpose 
of  reflecting  the  light  downward  and  protecting 
the  feeding  mechanism  of  the  lamp  from  the 
weather. 

The  crater  in  the  positive  carbon  is  the  main 
Source  of  light  in  the  arc  lamp.  When  an  area 


PRIMER  OF  PRIMERS.  381 

below  the  lamp  is  to  be  lighted,  it  is,  therefore, 
necessary  to  make  the  upper  carbon  the  positive 
carbon. 

When  arc  lamps  are  required  to  burn  for  a  greater 
number  of  hours  than  can  be  maintained  by  a  single 
pair  of  carbon  electrodes,  an  all-night  lamp,  contain- 
ing two  pairs  of  carbons  is  employed.  In  this  de- 
vice, when  one  pair  is  consumed,  the  current  is  au- 
tomatically shifted  to  the  other  pair. 

In  the  Jablochkoff  candle  the  two  carbons  are 
placed  parallel  to  one  another  and  separated  by  kao- 
lin or  some  other  suitable  insulating  material.  A 
Jablochkoff  candle  employs  an  alternating  current 
so  as  to  insure  a  uniformity  in  the  consumption  of 
the  two  carbons,  and  thus  burn  them  both  down  at 
an  even  rate. 

The  carbon  elec'trodes  employed  in  arc  lighting 
are  formed  of  artificial  carbon.  A  mixture  of 
powdered  coal  and  charcoal,  mixed  into  a  paste  with 
tar  or  some  other  carbonizable  liquid,  is  molded  by 
hydraulic  pressure,  dried  and  subsequently  carbon- 
ized while  out  of  contact  with  the  air.  The  carbon 
sticks  are  generally  covered  by  an  electrolytic  de- 
posit of  copper. 

The  unsteadiness  of  the  arc  light  is  due  mainly: 

(1.)  To  unsteadiness  in  the  driving  power. 


382  ELECTRICAL  MEASUREMENTS. 

(2.)  To  imperfections  in  the  feeding  mechanism 
of  the  lamp. 

(3.)  To  impurities  in  the  carbons. 

An  incandescent  lamp  consists  essentially: 

(1.)  Of  the  incandescing  filament  or  conductor. 

(2.)  Of  the  inclosing  glass  chamber. 

(H.)  Of  the  leading-in  wires. 

(4.)  Of  the  device  for  supporting  the  filament  in- 
side the  glass  chamber  and  connecting  it  to  the  lead- 
ing in  wires  or  conductors. 

(5.)  Of  the  base  containing  contact  points  to 
which  are  connected  the  leading-in  wires. 

(G.)  Of  a  socket  containing  contact  points  to 
which  are  connected  the  terminals  of  the  leads  that 
furnish  the  current  to  the  lamp. 

The  following  steps  are  essential  in  order  to  pre- 
pare the  carbon  strips  from  the  bamboo. 

(1.)  The  cutting  and  shaping  of  the  bamboo  fila- 
ment. 

(2.)  Carbonizing  the  filament  while  out  of  contact 
with  air. 

(3.)  Submitting  the  carbonized  filament  to  the 
flashing  process,  whereby  it  is  rendered  electrically 
homogeneous  throughout  its  entire  length. 

(4.)  Properly  mounting  and  connecting  the  car- 
bonized filament  to  the  leading-in  wires. 


PRIMER  OF  PRIMERS.  383 

(5.)  Driving  the  occluded  gases  out  of  the  fila- 
ment by  electrically  heating  it  while  in  the  lamp 
chamber  during  the  process  of  exhaustion. 

(6.)  The  hermetical  sealing  of  the  lamp. 

The  exhaustion  required  in  .  order  to  obtain  a 
vacuum  in  the  lamp  chamber  is  generally  com- 
menced by  the  action  of  a  mechanical  pump  and 
completed  by  the  action  of  a  mercury  pump. 

Incandescent  lamps  are  generally  connected  to  the 
mains  in  multiple-arc  or  in  multiple-series.  In  such 
cases  economy  of  distribution  is  best  obtained  when 
the  resistance  of  each  of  the  lamps  is  high. 

Incandescent  lamps  are' sometimes  connected  to 
the  line  wire  or  conductor  in  series  ;  in  such  cases 
the  electric  resistance  of  each  lamp  is  low. 

In  multiple-connected  incandescent  electric  lamps, 
the  cutting  out  of  a  single  lamp  does  not  affect  the 
other  lamps  in  the  circuit.  Such  circuits  are  pro- 
tected from  the  presence  of  abnormally  large  cur- 
rents by  placing  in  them  safety  fuses  or  strips,  which 
fuse  and  break  the  circuit  on  the  passage  of  a  cur- 
rent slightly  less  than  that  which  the  wire  form- 
ing the  circuit  can  stand  without  injury.  *  Such 
safety  fuses,  therefore,  protect  the  wire  circuits 
and  the  electro-receptive  devices  connected  there- 
with from  excessive  currents. 


384  ELECTRICAL  MEASUREMENTS. 

The  life  of  an  incandescent  lamp  is  rated  by  the 
number  of  hours  during  which  the  lamp  is  capable 
of  acting  as  an  effective  source  of  light.  The  failure 
of  a  lamp  to  properly  operate  may  arise  either  from 
the  breaking  of  the  filament  or  from  a  loss  of  trans- 
parency of  the  lamp  chamber.  This  decrease  of 
transparency  may  arise  either  from  the  settling  of 
dirt  on  the  outside  of  the  lamp  chamber,  or  from  an 
accumulation  of  volatilized  metal  or  carbon  on  the 
inside. 

When  a  current  of  electricity  flows  alternately  in 
opposite  directions  it  is  called  an  alternating  current 
in  contradistinction  to  a'direct  or  continuous  current 
that  continually  flows  in  one  and  the  same  direction. 

In  alternating  currents  the  electromotive  forces 
producing  the  currents  are  alternately  directed  in 
opposite  directions.  Their  values  may  be  represented 
by  means  of  a  curve. 

Two  or  more  alternating  currents  are  said  to  pos- 
sess the  same  phase  when  they  are  simultaneously 
similarly  directed.  Alternating  currents  are  said  to 
possess  the  same  period  when  their  number  of  alter- 
nations per  second  is  the  same.  They  are  said  to  be 
in  synchronism  with  each  other  when  their  electro- 
motive forces  produce  currents  in  the  same  direc- 
tion and  for  the  same  length  of  time. 


PRIMER  OF  PRIMERS.  385 

When  two  alternating  current  dynamos  are  con- 
nected in  series  to  the  same  leads  they  tend  to  drag 
each  other  into  opposite  phases  and  so  produce  no 
current.  When  connected  to  the  same  leads  in  par- 
allel they  tend  to  pull  each  other  into  synchronism. 
Series  connection,  or  coupling  of  alternators,  is 
therefore  impracticable. 

The  following  peculiarities  are  possessed  by  alter- 
nating currents: 

(1.)  The  currents  undergo  regular  changes  in 
direction. 

(2.)  The  currents  undergo  regular  changes  in 
strength. 

(3.)  The  peculiarities  of  alternation,  either  in 
direction  or  in  strength,  during  one  complete  alter- 
nation, that  is,  during  one  complete  to-aud-fro  mo- 
tion, are  regularly  repeated  during  any  subsequent 
complete  alternation  or  to-and-fro  motion  ;  in  other 
words,  such  motions  are  simple-periodic  or  simple- 
harmonic  motions. 

In  some  alternating  currents  the  motions,  though 
regularly  recurring,  are  more  complex  in  nature  and 
are,  therefore,  complex-harmonic  motions. 

In  alternating  currents  the  induction  of  the  cur- 
rent on  itself,  that  is,  its  self-induction  or  its  induc- 
tance, is  very  marked,  for  the  current  is  constantly 


386  ELECTRICAL  MEASUREMENTS. 

undergoing  changes  both  in  strength  and  in  direc- 
tion. 

The  resistance  of  any  circuit  to  the  passage  of  a 
current  through  it  is  called  its  impedance.  In  the 
case  of  an  alternating  current  the  impedance  is  equal 
to  the  sum  of  the  ohmic  resistance  of  the  circuit  and 
its  inductance. 

In  any  ordinary  direct  or  continuous  current  circuit 
the  current  strength  passing  at  any  moment  is  correctly 

IT 

represented  by  the  formula  C  —  -^  ;  in  which  case  0, 

Ji 

is  the  current  in  amperes,  E,  the  electromotive  force 
in  volts,  and  R,  the  resistance  in  ohms.  In  a  simple 
periodic  or  alternating  current  the  average  current 
strength  equals  the  average  impressed  electromotive 
force  divided  by  the  impedance.  The  impedance 
equals  the  square  root  of  the  sum  of  the  squares 
of  the  inductive  resistance  and  the  ohmic  resist- 
ance. 

When  a  continuous  current  is  passed  through  a 
conductor,  as  soon  as  the  current  becomes  steady 
the  current  density  is  the  same  in  all  cross-sections 
of  the  conductor.  When,  however,  an  alternating 
current  is  passed  through  a  conductor,  the  current 
density  is  greatest  near  the  surface,  and,  if  the 
rapidity  of  the  alternation  be  sufficiently  great,  the 


PRIMER  OF  PRIMERS.  387 

central  portions  of  the  conductor  are  nearly  free 
from  current. 

It  is  now  generally  believed  that  the  old  concep- 
tion of  a  current  passing  through  the  mass  of  a  con- 
ductor needs  modification.  The  electric  energy  is 
now  regarded  as  passing  through  the  dielectric  or 
non  conducting  space  outside  of  the  conductor,  and 
as  being  rained  down  on  the  surface  of  the  conduc- 
tor. 

The  discharge  of  a  Leyden  jar  partakes  of  the 
nature  of  rapidly  alternating  discharges ;  conse- 
quently, when  passed  through  the  primaries  of  in- 
duction coils  such  discharges  produce  by  induction 
rapidly  alternating  currents  in  the  secondaries  of 
such  coil. 

The  phenomena  of  the  alternative  path  of  a  dis- 
ruptive discharge  can  be  explained  by  the  oscillatory 
or  rapidly  alternating  character  of  such  discharges. 

The  so-called  anomalous  magnetization,  produced 
by  the  discharge  of  a  Leyden  jar  through  a  magnet- 
izing spiral,  is  caused  by  the  rapidly  alternating  char- 
acter of  such  discharges. 

Rapidly  alternating  currents  passed  through  con- 
ductors produce  by  induction  rapidly  alternating 
currents  in  neighboring  conductors.  The  effects  of 
such  induction  can  be  avoided  by  plates  of  metal 


388  ELECTRICAL  MEASUREMENTS. 

placed  between  the  conductors,  because  such  plates 
have  currents  produced  in  them  and  thus  protect 
the  conductors  from  them.  This  action  is  called 
screening. 

A  system  for  the  distribution  of  alternating 
currents  of  electricity  includes  : 

(1.)  An  alternating  current  source. 

(2.)  A  line  wire  or  conductor  arranged  in  a  me- 
tallic circuit. 

(3.)  Transformers  for  changing  or  transforming 
the  current  and  electromotive  force. 

(4.)  Electro-receptive  devices  placed  in  the  circuit 
of  the  secondary  coils  of  the  transformers. 

In  a  system  of  alternating  current  distribution 
rapidly  alternating  currents,  sent  over  the  line,  pass, 
through  the  primary  coils  of  an  instrument  called  a 
transformer  placed  in  the  same  circuit,  and  produce 
in  the  secondary  coils  of  such  transformer  rapidly 
alternating  currents,  which  differ  both  in  difference 
of  potential  and  in  current  strength  from  those  of 
the  primary. 

The  various  systems  devised  for  the  use  of  alter- 
nating currents  can  be  arranged  under  two  heads  ; 
namely: 

(1.)  A  system  of  constant  potential  distribution 
in  which  the  primaries  of  the  induction  coils  are 


PRIMER  OF  PRIMERS.  389 

connected  in  multiple  to  leads  maintained  at  a  con- 
stant difference  of  potential. 

(2.)  A  system  of  constant  current  distribution 
in  which  the  primary  coils  are  connected  in  series  to 
a  metallic  circuit. 

The  greater  the  electromotive  force  impressed  upon 
a  circuit  of  limited  area  of  cross-section  the  smaller 
the  amount  of  energy  lost  in  transmitting  a  required 
amount  of  energy  over  such  circuit. 

The  advantages  of  sending  over  aline  wire  or  con- 
ductor, a  current  of  higher  potential  than  can  be 
used  in  the  distribution  circuit,  and  subsequently 
lowering  or  lessening  such  potential  by  the  use  of 
transformers,  renders  the  use  of .  such  alternating 
currents  of  great  value  in  the  economical  distribu- 
tion of  electric  energy,  as  employed  in  systems  of 
incandescent  lamps. 

The  transformers  generally  employed  with  alter- 
nating currents  in  connection  with  incandescent 
lamps,  are  of  the  type  known  as  step-down  trans- 
formers ;  that  is,  transformers  in  which  a  great 
length  of  comparatively  thin  wire,  is  used  in  the 
primary,  and  a  smaller  length  of  comparatively  thick 
wire  in  the  secondary. 

In  step-down  transformers  a  current  of  great  dif- 
ference of  potential  and  small  current  strength  is 


390  ELECTRICAL  MEASUREMENTS. 

transformed  into  a  current  of  small  difference  of  po- 
tential and  great  current  strength. 

When  transformers  are  connected  to  leads  main- 
tained at  a  constant  difference  of  potential,  the  cur- 
rent that  flows  through  the  mains  is  automatically 
regulated  by  reason'of  the  self-induced  counter-elec- 
tromotive force  set  up  in  the  primary,  so  as  to  meet 
the  requirements  of  the  load  placed  on  the  mains, 
when  new  electro -receptive  devices  are  put  in  con^ 
nection  with  or  removed  from  such  mains. 

In  actual  practice  this  regulation  is  not  strictly 
automatic,  and  special  devices  are  required  in  order 
to  prevent  a  drop  of  potential  occurring'  on  the 
mains  or  excessive  variations  in  the  number  of  elec- 
tro-receptive devices  placed  thereon. 

In  systems  of  alternating  current  distribution  the 
source  consists  of  an  alternating  current  dynamo- 
electric  machine  or  battery  of  dynamo-electric  ma- 
chines. 

The  commercial  alternating  current  dynamo-elec- 
tric machines  required  to  produce  a  high  rate  of 
alternation  are  multipolar  ;  that  is,  they  possess  more 
than  a  single  pair  of  field  magnet  poles. 

In  a  system  of  distribution  by  means  of  alterna- 
ting currents,  in  order  to  protect  the  consumption 
circuit  from  the  high  potential  currents  sent  through 


PRIMER  OF  PRIMERS.  391 

the  primaries,  the  secondaries  should  be  highly  insu- 
lated from  the  primaries. 

The  passage  of  an  alternating  current  through  an 
incandescent  electric  lamp  produces  an  alternate  in- 
crease and  decrease  in  the  temperature  of  the  lamp, 
and  consequently  an  alternate  increase  and  decrease 
in  the  brightness  of  the  emitted  light.  Such  lamps, 
however,  produce  a  steady  light  provided  the  rate 
of  alternation  is  sufficiently  high. 

In  a  system  of  direct  or  continuous  current  dis- 
tribution by  means  of  transformers,  devices  called 
motor-generators  are  employed  in  order  to  convert 
the  continuous  current  of  high  potential  sent  over 
the  main  line  or  conductor,  into  the  character,  ^of 
current  required  for  use  in  the  consumption  circuit. 
In  some  cases,  however,  devices  called  disjunctors 
are  employed  to  rapidly  and  periodically  reverse  the 
current  so  as  to  feed  the  transformers,  by  means  of 
which  the  current  is  transformed  to  one  of  smaller 
potential  and  greater  current. 

When  two  alternating  current  dynamos  are  con- 
nected in  series  so  as  to  form  a  single  source,  the  two 
machines  soon  pull  each  other  into  opposite  phases ; 
when,  however,  they  are  connected  in  multiple,  even 
though  out  of  synchronism  they  soon  pull  each  other 
into  synchronism. 


392  ELECTRICAL  MEASUREMENTS. 

A  choking  or  reaction  coil  consists  of  a  coil  of  in- 
sulated wire  wound  on  a  core  of  soft  iron  wire. 
Such  a  coil  acts  by  its  self-induction  to  choke  off  an 
alternating  current  endeavoring  to  pass  through  it 
with  much  less  loss  of  power  than  if  an  ohmic  re- 
sistance were  used.  The  higher  the  rapidity  of  alter- 
nation the  greater  is  the  choking  effect  of  a  given  coil. 

A  choking  coil  employed  for  the  purpose  of  auto- 
matically regulating  the  intensity  of  the  light 
emitted  by  an  incandescent  lamp  is  called  a  dimmer. 

When  the  rate  of  alternation  becomes  exceeding- 
Iv  high,  alternating  currents  present  numerous  phe- 
nomena which  differ  in  a  marked  manner  from 
those  possessed  by  currents  of  but  moderately  high 
frequency. 

Among  such  differences  are  : 

(1.)  When  sent  through  electro-magnets  the  alter- 
nate attractions  and  repulsions  of  the  armatures  dis- 
appear, and  apparently  nothing  remains  but  at- 
traction. 

(2.)  Substances  which  act  as  insulators  for  ordi- 
nary currents  act  as  conductors  for  alternating  cur- 
rents of  enormous  frequency. 

(3.)  Substances  which  act  as  conductors  for  ordi- 
nary currents  will  not  permit  alternating  currents 
of  very  high  frequency  to  pass  through  them. 


PRIMER  OF  PRIMERS.  393 

(4.)  The  physiological  effects  of  alternating  cur- 
rents of  high  frequency  are  much  less  severe  than 
are  those  of  but  moderate  frequency. 

By  employing  alternating  currents  of  enormously 
high  frequency,  and  sending  them  through  the  pri- 
maries of  peculiarly  constructed  induction  coils, 
Nikola  Tesla  obtains  a  great  variety  of  unusual 
and  curious  electric  discharges.  Among  some  of  the 
most  important  of  these  are  : 

(1.)  The  sensitive-thread  discharge. 

(2.)  The  flaming  discharge. 

(3.)  The  streaming  discharge. 

(4.)  The  brush-and-spray  discharge. 

(5.)  The  luminous-disc-shaped  discharge. 

(6.)  The  rotating-brush  discharge. 

Tesla  has  devised  a  new  form  of  electric  lamp  which 
may  be  termed  an  electric  bombardment  lamp,  in 
which  the  light  is  obtained  from  the  intense  molec- 
ular bombardment  of  gaseous  molecules  under  the 
influence  of  electric  discharges  of  enormously  high 
frequency. 

Electric  bombardment  lamps  may  be  rendered 
luminous  when  subjected  to  the  electrostatic  thrusts 
of  discharges  of  enormous  frequency  when  but  a 
single  pole  is  connected  to  the  source  of  such  dis- 


394  ELECTRICAL  MEASUREMENTS. 

charges,  or  even  when  no  poles  whatever  are  con- 
nected to  such  source. 

Among  the  many  forms  of  electric  bombardment 
lamps  devised  by  Tesla  may  be  mentioned  the  ball 
incandescent  electric  lamp,,  and  the  straight-filament 
incandescent  electric  lamp. 

By  causing  the  oscillatory  discharges  of  high  po- 
tential obtained  from  a  condenser  to  pass  through 
the  primaries  of  induction  coils,  Elihu  Thomson  has 
obtained  discharges  from  their  secondaries  of  enor- 
mously high  potential  and  frequency.  Some  of  these 
discharges  readily  pass  through  thirty  inches  of  air, 
at  an  estimated  difference  of  potential  of  five  hun- 
dred thousand  volts. 

The  principles  of  electro-dynamic  induction  were 
discovered  by  Faraday  about  1831.  When  a  con- 
ductor is  caused  to  cut,  or  is  cut  by,  the  lines  of  mag- 
netic force,  it  has  differences  of  potential  produced  in 
it  by  what  is  called  electro-dynamic  induction. 

In  electro-dynamic  induction  a  motion,  therefore, 
is  necessary  either  on  the  part  of  the  conductor,  so  as 
to  cause  it  to  cut  the  lines  of  magnetic  force,  or  on 
the  part  of  the  lines  of  force,  so  as  to  cause  them  to 
cut  the  conductor;  or,  in  other  words,  the  magnetic 
field  may  be  either  stationary  or  in  motion,  but  one 
or  the  other  must  be,  and  both  may  be  in  motion. 


PRIMER  OF  PRIMERS.  395 

When  the  strength  of  a  current  passing  through  a 
conductor  increases,  the  circular  lines  of  force  sur- 
rounding such  conductor  increase  in  number  and  ex- 
pand or  move  outward.  When  the  current  strength 
decreases  the  circular  lines  of  force  decrease  in  num- 
ber and  contract  or  move  inward. 

Neighboring  conductors,  so  placed  as  to  be  cut  by 
these  expanding  and  contracting  lines  of  force,  have 
differences  of  potential  produced  in  them  by  electro- 
dynamic  induction. 

Electro-dynamic  induction  may  be  produced  : 

(1.)  By  moving  a  conductor  through  the  lines  of 
magnetic  force  so  as  to  cut  them. 

(2.)  By  placing  a  conductor  so  as  to  be  cut  by  the 
expanding  or  contracting  lines  of  force. 

There  are  four  varieties  of  electro-dynamic  induc- 
tion ;  namely, 

(1.)  Self-induction  or  inductance. 

(2.)  Mutual-induction  or  voltaic-current  induc- 
tion. 

(3.)  Magneto-electric  induction. 

(4.)  Electro-magnetic  induction. 

Magneto-electric  induction  and  electro-magnetic 
induction  are  sometimes 'called  dynamo-electric  in- 
duction. 

In  self-induction  the  expanding  or  contracting 


396  ELECTRICAL  MEASUREMENTS. 

lines  of  force,  produced  by  variations  in  the  current 
strength  in  a  given  circuit,  are  caused  to  cut  parts 
of  the  circuit  and  thereby  induce  differences  of 
potential  therein. 

Currents  are  produced  in  a  circuit  by  self-induc- 
tion both  on  starting  and  stopping  a  current  in  such 
circuit. 

These  induced  currents  are  called  extra  currents- 

The  extra  currents  flow  in  the  opposite  direction  to 

the  inducing  current  on  the  completing  or  closing  of 

the  circuit  and  in  the  same  direction  on  the  opening 

or  breaking  of  the  circuit. 

Since  both  in  self-induction  and  in  mutual  induc- 
tion the  differences  of  potential  are  produced  by  ex- 
panding and  contracting  lines  of  magnetic  force,  and 
since  such  expansions  and  contractions  occur  only 
while  the  current  strength  is  changing,  self-induc- 
tion or  mutual  induction  is  produced  only  while  the 
current  is  undergoing  a  change  in  strength.  As  soon 
as  a  steady  flow  is  established  through  the  conductor 
the  effects  of  induction  cease. 

In  mutual-induction  the  expanding  and  contract- 
ing lines  of  force  produced  by  rapidly  varying  the 
current  strength  in  one  circuit  cut  or  pass  through 
a  neighboring  circuit  and  induce  difference  of  poten- 
tial therein.  Such  differences  of  potential  are  di- 


PRIMER  OF  PRIMERS.  397 

rected  in  one  direction  as  the  lines  of  force  expand 
or  move  outward  from  the  inducing  circuit,  and  in 
the  opposite  direction  as  they  contract  or  move  in- 
ward toward  such  circuit. 

The  effects  of  mutual-induction  are  utilized  in 
those  forms  of  electro-receptive  devices  termed  in- 
duction coils. 

In  an  induction  coil  a  rapidly  alternating  current 
sent  through  a  circuit  called  the  primary  circuit, 
produces  by  induction  a  rapidly  alternating  current, 
in  a  neighboring  circuit  called  the  secondary  circuit. 

In  magneto-electric  induction  a  conductor  is 
moved  toward  the  field  of  a  permanent  magnet  so 
as  to  cut  the  lines  of  force,  or  such  field  is  caused  to 
pass  across  the  conductor  by  moving  the  magnet 
past  the  conductor. 

In  electro-magnetic  induction  a  conductor  is 
moved  past  an  electro-magnet  so  as  to  cause  the 
conductor  to  cut  or  to  be  cut  by  the  lines  of  mag- 
netic force  of  the  magnet,  or  vice  versa. 

Magneto-electric  induction  and  electro-magnetic 
induction  are,  therefore,  in  reality  one  and  the  same 
variety  of  electro-dynamic  induction  and  are  some- 
times called  dynamo-electric  induction. 

The  following  general  principles  can  be  applied 
to  all  cases  of  dynamo-electric  induction  ;  namely  : 


398  ELECTRICAL  MEASUREMENTS. 

(1.)  Any  increase  in  the  number  of  lines  of  mag- 
netic force  which  pass  through  a  circuit  produces  an 
inverse  current  in  that  circuit  ;  that  is,  a  current 
flowing  in  the  opposite  direction  to  that  producing 
the  lines  of  force. 

•  (2.)  Any  decrease  in  the  number  of  lines  of  force 
passing  through  a  circuit  produces  a  direct  current 
in  that  circuit  j  that  is,  a  current  flowing  in  the 
same  direction  as  that  producing  the  magnetic  field. 

(3.)  The  strength  of  the  induced  current,  or,  more 
correctly,  the  difference  of  potential  produced,  is  pro- 
portional to  the  rate  of  increase  or  decrease  in  the 
number  of  lines  of  force  passing  through  the  circuit. 

Induction  coils  are  forms  of  electro-receptive  de- 
vices by  means  of  which  rapidly  alternating  cur- 
rents, passing  through  a  primary  circuit,  induce  by 
electro-dynamic  induction  currents  in  a  neighboring 
secondary  circuit. 

Induction  coils  are  sometimes  called  converters  or 
transformers.  The  latter  term  is  more  frequently 
employed. 

Transformers  can  be  divided  into  the  two  general 
classes  of  step-up  transformers  and  step-down  trans- 
formers. 

In  step-up  transformers  the  length  or  number  of 
turns  of  the  primary  circuit  is  less  than  the  length 


PRIMER  OF  PRIMERS.  399 

or  number  of  turns  of  the  secondary  circuit.  When, 
therefore,  a  difference  of  potential  is  applied  at  the 
terminals  of  the  primary  circuit,  a  higher  difference 
of  potential  is  produced  by  induction  at  the  termi- 
nals of  the  secondary  circuit. 

In  step-down  transformers  the  length  or  number 
of  turns  of  the  primary  circuit  is  greater  than  that 
of  the  secondary  circuit.  When,  therefore,  a  differ- 
ence of  potential  is  applied  to  the  terminals  of  the 
primary  circuit,  a  lower  difference  of  potential  is  in- 
duced by  dynamo-electric  induction  in  the  secondary 
circuit. 

In  the  distribution  of  electricity  by  means  of  al- 
ternating currents  a  step-down  transformer  is  gener- 
ally employed. 

In  induction  coils,  as  first  constructed,  the  length 
of  the  primary  was  much  less  than  that  of  the  sec- 
ondary ;  that  is,  such  induction  coils  were  step-up 
transformers.  For  this  reason  a  step-down  trans- 
former is  frequently  called  an  inverted  induction 
coil. 

In  any  transformer  the  direction  of  the  currents 
produced  in  the  secondary  circuit  are  as  follows, 
namely  : 

(1.)  Opposite  in  direction  to  that  of  the  current 
in  the  primary  circuit  on  making  or  completing  such 


400  ELECTRICAL  MEASUREMENTS. 

circuit ;  that  is,  when  the  lines  of  magnetic  force  of 
such  circuit  are  expanding. 

(2.)  In  the  same  direction  as  the  currents  in  the 
primary  circuit  on  breaking  or  opening  such  circuit ; 
that  is,  when  the  lines  of  magnetic  force  of  such  cir- 
cuit are  contracting. 

In  any  transformer  the  relative  differences  of  po- 
tential in  the  primary  and  secondary  circuits  are 
proportional  to  the  relative  lengths  or  number  of 
turns  of  such  circuits. 

If,  for  example,  the  length  of  the  secondary  is  fifty 
times  the  length  of  the  primary,  the  difference  of 
potential  induced  in  the  secondary  will  be  fifty  times 
that  of  the  difference  of  potential  impressed  on  the 
primary. 

Disregarding  losses  by  conversion,  the  electric  en- 
ergy produced  in  the  secondary  by  induction  is  equal 
in  amount  to  the  electric  energy  in  the  primary. 

Representing  the  electric  energy  as  the  product  of 
the  current  in  amperes  by  the  difference  of  potential 
in  volts,  C E,  equals  the  energy  of  the  primary,  and 
C'  E',  the  energy  of  the  secondary  ;  in  other  words, 
therefore,  C  E  =  C'  E'.  As  much,  therefore,  as 
the  current  strength  in  the  secondary  is  increased 
over  that  in  the  primary,  the  electromotive  force  in 
the  secondary  must  be  decreased,  and  vice  versa. 


PRIMER  OF  PRIMERS.  401 

The  principles  of  self-induction  and  mutual-induc- 
tion were  discovered  by  Professor  Joseph  Henry. 

In  the  case  of  transformers  the  energy  which  ap- 
pears in  the  secondary  circuit  is  somewhat  less  than 
that  expended  in  the  primary  circuit  for  the  follow- 
ing reasons: 

(1.)  On  account  of  the  specific  inductive  capacity 
of  the  medium  between  the  two  circuits. 

(2.)  On  account  of  hysteresis,  or  magnetic  fric- 
tion. 

(3.)  From  loss  of  energy  in  the  primary  circuit 
from  heating  it. 

(4.)  Energy  similarly  expended  in  the  secondary 
circuit. 

In  the  pyro-magnetic  generator  electric  currents 
are  obtained  by  a  species  of  dynamo-electric  induc- 
tion. 

A  dynamo-electric  machine  consists  of  any  com- 
bination of  parts  by  means  of  which  mechanical  en- 
ergy is  converted  into  electrical  energy  by  dynamo- 
electric  induction,  by  the  cutting  of  lines  of  mag- 
netic force  by  conductors. 

A  dynamo-electric  machine  is  sometimes  more 
broadly  defined  to  be  any  machine  for  converting 
energy  in  the  form  of  mechanical  power  into  electric 
currents,  or  vice  versa,  by  causing  conductors  to  move 


402  ELECTRICAL  MEASUREMENTS. 

across  a  magnetic  field  or  by  varying  -a  magnetic 
field  in  their  neighborhood. 

Dynamo-electric  machines  consist  essentially  of 
the  following  parts  ;  namely, 

(1.)  A  moving  part  called  the  armature,  contain- 
ing conductors  in  which  differences  of  potential  are 
produced.  The  armature  is  sometimes  stationary. 

(2.)  The  field  magnets  which  produce  the  mag- 
netic field. 

(3.)  The  pole  pieces  which  act  to  concentrate  the 
magnetic  field  on  the  armature. 

(4.)  The  commutator,  by  means  of  which  the 
currents  produced  in  the  armature  are  caused  to  flow 
in  one  and  the  same  direction. 

(5.)  The  collecting  brushes  that  rest  on  the  com- 
mutator cylinder  and  carry  off  the  current  produced 
by  the  difference  of  potential  in  the  armature. 

The  armatures  are  made  in  a  great  variety  of 
forms,  such  as  drum-armatures,  ring-armatures, 
radial-armatures,  pole-armatures. 

The  field  magnet  cores  are  made  of  massive  solid 
iron  as  soft  as  possible;  the  pole  pieces  of  the  field 
magnets  are  also  made  of  soft  iron,  and  may  be 
laminated  in  order  to  prevent  the  loss  of  energy  from 
the  production  in  them  of  currents  called  eddy 
currents. 


OP  PRIMERS.  403 

The  collecting  brushes  are  formed  of  bundles, 
strips  or  plates  of  copper  wire  suitably  soldered  to- 
gether, or  are  formed  of  various  compositions  of 
carbon  and  graphite. 

The  armature  is  made  of  a  laminated  core  of  soft 
iron  in  which  coils  of  insulated  wire  are  placed.  • 

When  a  coil  of  wire  is  rotated  in  the  magnetic 
field  formed  by  the  two  opposite  magnet  poles,  dif- 
ferences of  potential  are  generated  in  such  coil  which 
produce  currents  that  change  their  direction  twice 
during  every  complete  revolution. 

Such  currents  can  be  made  to  flow  in  one  and 
the  same  direction  through  a  circuit  external  to  the 
armature  by  means  of  devices  called  commutators. 

Dynamo-electric  machines  may  be  divided  into 
different  classes ;  namely, 

(1.)  Bi-polar  and  multi-polar  machines,  accord- 
ing to  the  number  and  disposition  of  the  field  mag- 
nets. 

(2.)  Into  self-excited  and  separately-excited  ma- 
chines, -according  to  the  manner  in  which  the  excita- 
tion of  the  machine  is  obtained. 

(3.)  Into  simple  and  compound-wound  machines, 
according  to  the  number  and  arrangement  of  sepa- 
rate circuits  on  the  field  magnet's  coils. 

(4.)  Into  various  classes,  according  to  the  charac- 


404  ELECTRICAL  MEASUREMENTS. 

ter  of  the  connections  between  the  circuit  of  the 
field  magnets,  the  armature  circuit,  and  the  cir- 
cuit external  to  the  machine. 

(5. )  Into  various  classes,  according  to  the  character 
of  the  armature  winding  or  the  shape  of  the  arma 
ture  itself. 

In  the  series  dynamo  the  circuits  of  the  field  mag- 
nets and  the  external  circuit  are  connected  in 
series  with  the  armature  circuit  so  that  the  entire 
armature  current  passes  through  the  field  magnet 
coils. 

In  the  shunt  dynamo  the  field  magnet  coils  are 
placed  in  a  shunt  to  the  armature  terminals  or  the 
external  circuit,  so  that  only  a  portion  of  the  current 
generated  passes  through  the  field  magnet  coils,  but 
all  the  difference  of  potential  acts  at  the  terminals 
of  the  field  circuit. 

In  a  separately-excited  dynamo  the  field  magnet 
coils  have  no  connection  with  the  armature  coils, 
but  receive  their  current  from  a  separate  machine 
or  source. 

In  the  series-and-separately-excited  dynamo  the 
field  magnets  are  wound  with  two  separate  coils,  one 
of  which  is  connected  in  series  to  the  external  cir- 
cuit, and  the  other  with  some  source  external  to  the 
machine,  by  means  of  which  it  is  separately  excited. 


PRIMER  OF  PRIMERS.  405 

In  the  series-and-shunt-wound  dynamo  the  field 
magnet  cores  are  wound  with  two  separate  coils,  one 
of  which  is  placed  in  series  with  the  armature  and 
the  external  circuit,  and  the  other  in  shunt  to  the 
external  circuit.  Such  machines  are  called  com- 
pound-wound machines. 

Electro-dynamics  is  that  branch  of  electric  science 
which  treats  of  the  action  of  an  electric  current  on 
itself,  on  another  current,  or  on  a  magnet.  The 
term  is  used  in  contradistinction  to  electro-statics. 

Ampere  established  the  following  general  princi- 
ples of  electro-dynamics. 

(1.)  Parallel  circuits  through  which  electric  cur- 
rents are  flowing  in  the  same  direction  attract  each 
other. 

(2.)  Parallel  circuits  through  which  electric  cur- 
rents are  flowing  in  opposite  directions  repel  each 
other. 

(3.)  Circuits  placed  so  as  to  mutually  intersect  at- 
tract each  other  when  the  currents  through  them 
flow  toward  or  from  the  point  of  intersection,  but 
repel  each  other  when  the  current  through  one  of 
them  approaches  and  that  through  the  other  recedes 
from  the  point  of  intersection . 

Electro-dynamic  attractions  and  repulsions  are  pro- 
duced by  the  action  of  magnets  on  movable  circuits 


406  ELECTRICAL  MEASUREMENTS. 

as  well  as  by  the  action  of  the  circuits  on  one  another. 
The  direction  of  the  motion  produced  can  be  deter- 
mined by  reference  to  the  direction  of  the  amperian 
currents  that  are  assumed  to  cause  the  magnetism. 

A  solenoid  consists  of  a  coil  of  insulated  wire, 
which  acquires  the  properties  of  a  magnet  when 
traversed  by  an  electric  current. 

Unlike  poles  of  a  solenoid  attract  each  other,  be- 
cause the  currents  which  produce  such  poles  flow  in 
the  same  direction  in  parts  of  the  circuit  lying  near- 
est to  each  other.  Like  poles  repel  each  other 
because  the  currents  producing  them  flow  in  opposite 
directions  in  parts  of  the  circuit  lying  nearest  to  each 
other. 

Currents  flowing  in  the  same  direction  through 
parallel  circuits  attract  each  other  because  their  ap- 
proached lines  of  magnetic  force  extend  in  opposite 
directions;  and  oppositely  directed  lines  of  magnetic 
force  attract  one  another. 

Currents  flowing  in  opposite  directions  through 
parallel  circuits  repel  one  another  because  their  ap- 
proached lines  of  magnetic  force  extend  in  the  same 
direction ;  and  similarly  directed  lines  of  magnetic 
force  repel  one  another. 

When  a  circuit  is  bent  on  itself  so  that  the  current 
in  one  part  of  the  circuit  flows  in  the  opposite  di- 


PRIMER  OF  PRIMERS.  407 

rection  to  that  in  the  remaining  part,  the  two  parts 
exert  no  force  of  magnetic  attraction  or  repulsion 
on  external  objects,  because  their  fields  neutralize 
one  another.  This  expedient  is  adopted  in  the 
winding  of  resistance  coils. 

An  electric  motor  consists  of  any  combination  of 
parts  by  means  of  which  electric  energy  is  con- 
verted into  mechanical  energy. 

As  generally  constructed,  electric  motors  convert 
electrical  energy  into  mechanical  energy  by  means 
of  the  attractions  and  repulsions  exerted  between 
the  magnetic  fields  of  electro-magnets,  or  between 
the  fields  of  electro-magnets  and  the  fields  produced 
by  currents  flowing  through  neighboring  conduc- 
tors. 

By  the  reversibility  of  a  dynamo-electric  machine 
is  meant  its  ability  to  operate  as  a  motor  when  a 
current  of  electricity  is  sent  through  its  circuit. 

The  following  analogies  exist  between  dynamo- 
electric  machines  and  electric  motors  : 

(1.)  When  mechanical  energy  is  applied  to  a  dy- 
namo, so  as  to  rotate  its  armature,  a  difference  of  po- 
tential is  produced  therein ;  conversely,  if  electric 
energy  in  the  form  of  a  current  be  sent  through  the 
armature  and  field,  it  will  rotate  and  produce  me- 
chanical energy. 


408  ELECTRICAL  MEASUREMENTS. 

(2.)  When  mechanical  energy  is  applied  to  a  suit- 
ably designed  dynamo,"  so  as  to  rotate  its  armature 
at  a  constant  speed,  the  electromotive  force  remains 
constant  irrespective  of  the  load,  or  irrespective  of 
the  current  produced  ;  conversely,  when  electric 
energy  is  applied  to  a  suitably  designed  motor,  if  a 
constant  difference  of  potential  is  maintained  at  its 
terminals  it  will  run  at  a  constant  speed  approxi- 
mately irrespective  of  the  load  placed  on  it. 

The  pull  produced  on  the  shaft  of  an  electric  mo- 
tor by  the  action  of  the  magnetic  fields,  or  the 
amount  of  turning  force  the  shaft  exerts,,  is  called  its 
torque.  The  efficiency  of  a  motor  is  the  ratio  be- 
tween the  electric  energy  required  to  turn  the  motor 
and  the  mechanical  energy  it  produces. 

The  brushes  on  an  electric  motor  require  to  be 
placed  in  a  different  position  from  those  on  a  dyna- 
mo in  order  to  avoid  excessive  sparking  at  the  com- 
mutator. 

In  both  cases  a  lead  is  given  to  the  brushes,  but 
in  the  case  of  the  motor,  in  order  to  obtain  the  po- 
sition of  least  sparking  this  lead  is  in  the  opposite 
direction  to  that  of  the  dynamo.  In  well  designed 
motors  the  amount  of  this  lead  is  inconsiderable. 

The  reversal  of  the  direction  of  motion  of  a  motor 
can  be  obtained  in  various  ways:* 


PRIMER  OF  PRIMERS.  409 

(1.)  By  reversing  the  connections  of  the  armature. 

(2.)  By  reversing  the  connections  of  the  field  mag- 
nets. 

When  an  electric  current  is  sent  through  a  motor 
the  direction  of  its  rotation  will  vary  with  the  man- 
ner in  which  its  armature  and  field  magnets  are  con- 
nected. 

When  a  series-dynamo  is  used  as  a  motor  it  will 
rotate  in  the  opposite  direction  to  that  in  which  it 
is  driven  as  a  generator,  unless  the  polarity  of  the 
field  only  is  reversed,  when  it  may  be  caused  to  rotate 
in  the  same  direction  as  it  does  when  driven  as  a 
generator. 

When  a  shunt  dynamo  is  used  as  a  motor  it  will 
rotate  in  the  same  direction  as  that  in  which  it  is 
driven  as  a  generator. 

When  a  compound-wound  dynamo  is  driven  as  a 
motor  it  will  rotate  in  a  direction  opposite  to  that  of 
its  motion  as  a  generator  when  its  series  coils  are 
more  powerful  than  its  shunt  coils,  and  in  the  same 
direction  when  its  shunt  coils  are  more  powerful 
than  its  series  coils. 

During  the  rotation  of  the  armature  of  a  motor, 
an  electromotive  force  is  produced,  as  its  wire  cuts 
the  lines  of  force  of  its  field,  that  is  oppositely  di- 
rected to  that  of  the  current  which  produces  the 


410  ELECTRICAL  MEASUREMENTS. 

motion.  This  electromotive  force  is  called  the 
counter  electromotive  force  of  the  motor,  and  acts 
like  a  resistance  and  opposes  the  passage  of  the 
driving  current.  As  the  speed  of  rotation  increases 
this  counter  electromotive  force  increases,  and  the 
current  required  to  drive  the  dynamo  becomes  less, 
until,  when  the  maximum  speed  is  reached,  the  driv- 
ing current  is  very  small. 

When  a  load  is  placed  on  a  motor,  its  speed  being 
reduced,  the  counter  electromotive  force  decreases, 
and  a  greater  driving  current  is  thus  permitted  to 
pass  through  it.  In  this  way  an  electric  motor  au- 
tomatically regulates  the  current  required  to  drive 
it.  An  electric  motor  performs  its  maximum  rate  of 
work  when  this  theoretical  efficiency  is  about  50  per 
cent. 

In  the  pyromagnetic  motor  the  motion  is  obtained 
by  the  attraction  which  magnetic  poles  exert  on  an 
unequally  heated  iron  disc. 

A  system  for  the  electric  transmission  of  power 
consists  essentially  of  the  following  parts  : 

(1.)  A  line  wire  or  conductor  connecting  two 
stations. 

(2.)  An  electric  source  or  battery  of  electric 
sources,  generally  in  the  form  of  dynamo-electric 
machines,  placed  at  one  of  the  stations  for  the  pur- 


PRIMER  OF  PRIMERS.  411 

pose  of  converting  mechanical  energy  into  electrical 
energy. 

(3.)  An  electro-receptive  device  or  devices,  placed 
in  the  circuit  of  the  line  wire  or  conductor,- gener- 
ally in  the  form  of  an  electric  motor,  for  the  purpose 
of  reconverting  the  electric  energy  into  mechanical 
energy. 

The  circuits  connecting  the  sources  with  the 
electro-receptive  devices  may  be  either  constant-cur- 
rent circuits  or  constant-potential  circuits. 

The  electric  transmission  of  power  possesses  the 
following  advantages  over  any  other  system,  namely  : 

(1.)  The  greater  distance  over  which  power  may 
be  economically  transmitted. 

(2.)  A  greater  economy. 

(3.)  The  utilization  of  sources  of  power  that 
would  otherwise  be  impracticable. 

(4.)  A  greater  simplicity  in  the  means  required 
to  connect  the  driving  power  with  the  driven 
mechanism. 

The  efficiency  of  the  electric  motor  over  that  of 
the  steam-engine  will  enable  the  electric  motor  to 
economically  replace  the  steam-engine  in  all  cases 
where  electricity  can  be  economically  produced  inde- 
pendently of  the  intervention  of  steam  power. 

The  electric  motor  can  advantageously  replace  the 


412  ELECTRICAL  MEASUREMENTS. 

steam-engine  as  a  prime  mover  under  the  following 
circumstances  : 

(1.)  AVhere  it  replaces  horse  power. 

(2.)  Wherever  available  water  power  exists. 

(3.)  In  all  places  where  a  small  amount  of  power 
is  required  by  a  sufficient  number  of  people  in  any 
locality,  and,  consequently,  where  a  single  steam- 
engine  can  be  employed  to  drive  a  dynamo  or  battery 
of  dynamos  and  so  supply  electric  power  to  a  com- 
paratively extended  area. 

The  various  systems  of  telegraphic  and  telephonic 
communication  offer  instances  of  the  transmission  of 
electric  power.  In  both  cases  mechanical  motions  at 
one  end  of  the  wire  are  reproduced  electrically  at 
the  other  end  of  the  wire  or  conductor. 

In  some  systems  for  the  transmission  of  electric 
power,  the  electric  energy  produced  at  one  end  of  a 
line  wire  or  conductor  is  utilized  not  only  at  the 
other  end,  but  at  intermediate  points  between  the 
ends.  Examples  of  this  are  seen  in  various  telpher- 
age systems,  porte-electric  systems  and  various  sys- 
tems of  electric  railroads. 

In  the  telpherage  system  a  carriage  suspended 
from  a  bare  line  wire  or  conductor  is  propelled  along 
it  by  the  action  of  an  electric  motor  which  takes 
from  the  line  the  electric  energy  required  to  drive  it. 


PRIMER  OF  PRIMERS.  413 

In  the  porte-electric  system  of  transmission  a  cyl- 
indrical car,  in  the  form  of  a  movable  core,  is  rapidly 
sucked  through  a  number  of  helical  coils  by  the  suc- 
cessive attractions  which  they  exert  on  the  car. 

Various  systems  have  been  proposed  for  the  elec- 
tric propulsion  of  railway  cars. 

These  systems  may  be  divided  into — 

(1.)  The  independent  system,  where  the  driving 
current  is  derived  from  primary  or  secondary  batter- 
ies placed  on  the  moving  car  ;  or 

(2.)  The  dependent  system,  where  the  driving 
power  is  taken  by  means  of  sliding  or  rolling  con- 
tacts from  conductors  placed  outside  the  car. 

The  dependent  system  of  motive  power  for  elec- 
tric railways  includes  : 

(1.)  The  underground  system. 
(2.)  The  surface  system. 
(3.)  The  overhead  system. 

In  all  dependent  systems,  since  the  driving  cur- 
rent is  taken  directly  from  the  line  wire  or  con- 
ductor by  rolling  or  sliding  contacts  that  move  over 
it,  such  wire  or  conductor  must  necessarily  be  bare 
or  uninsulated,  and  must,,  therefore,  be  suitably  sup- 
ported on  conveniently  placed  insulators. 

The  overhead  system,  so  far,  is  the  only  one  which 


414  ELECTRICAL  MEASUREMENTS. 

has  come  into  extended  public  use.  In  it  the  driv- 
ing current  is  taken  from  an  overhead  wire  or  con- 
ductor by  means  of  a  traveling  wheel  called  a  trolley. 


INDEX. 


A  CT1ON  of  commutator,  299 
•**•    Air  pump,  Sprengel's  men 

curial,  171,  172 
Alarm,  photo-electric,  116 
All-night  arc  lamp,  152, 153,  154 
Alternating  current,  phase  of, 

186 
dynamo-electric     machines, 

216,  217 
period  of,  183 
currents,  185 
distribution  of,  207 
curve  of  electromotive  force 

of,  186 

definition  of,  125 
peculiarities  of,  188 
synchronism  of.  187 
Alternative  path,  phenomena  of, 

196,  197 
Alternators,  parallel  connection 

of,  187 
series  connection   of  imprac 

ticable.  187 
Amalgamation  of  zinc  plate  of 

voltaic  cell,  92 

Ammeters  or  ampere  meters,  20 
Ampi-re,  apparatus  of,  13 

definition  of,  8 

Ampere  meters  or  ammeters,  20 
Ampere's  apparatus,  description 

of,  313 

Ampere's  laws  of  electro  dynam- 
ics, 311,  312 

stand,  description  of,  314 
Analogies  between  dynamVelec- 
tric  machines  and  electric 
motors,  329 


Analogous  pole.  119 
Angular  currents,  attraction  be- 
tween, 315 

repulsion  between,  316 
Animals,  effect  of  electricity  on, 

121 

Animals,  electricity  of,  121 
Anomalous  magnetism,  198 
Antilogous  pole,  119 
Apparatus,  Ampere's,  13 
Arc  lamp,  all-night,  152, 153, 154 
double-carbon,  153, 154 
hood  of,  156 
outrigger  for,  157 
carbon  holders  for,  148,  149 
clamping     or  clutching   de- 
vices for  lamp  rod  of,  149 
feeding  devices  of,  149 
forms  of,  151,  152 
operation  of    shunt   magnet 

in,  150 
various  constructions  of,  148, 

119 

Arc  light,  causes  for  unsteadi- 
ness of,  159 
traveling  of,  159 
Arc-like  or  flaming  discharge, 

231 
Arc,  voltaic,  carbon,  appearance 

of,  147. 148 
effect  of  temperature  on  light 

emitting  power  of,  146 
exhibition  of,  by  Davy,  145 
Armature,  drum,  290 
loop,  induction  in.  298 
of  dynamo  electric  machine, 


(415) 


416 


INDEX. 


Armature,  pole,  292 

ring,  291   • 

Arms  of  electric  bridge,  65 
Artificial  carbon  electrodes,  158 
parchment  for  incandescing 
filament  of   electric  lamp, 
174,  175 
Astatic  galvanometer,  17 

needle.  18 

Attracted  disc  electrometer,    47 
Attraction  between  angular  cur- 
rents, 315 
Automatic  photo-electric  switch, 

116 

regulation      by     compound- 
winding,  135 
regulation   of    transformers, 

213 

safety  device  for  series-con- 
nected arc  lamps.  151 
system  of  regulation,  Brush, 

131 

system  of  regulation,  Thom- 
son-Houston, 130 
Ayrton's  ohmmeter.  74,  75 

"DALANCE,  electric,  65 

-*    Balanced  reaction  coil,  223 
Ballistic  galvanometer,  30 
Barlow's  wheel,  328 
Base    of    incandescent    electric 

lamp,  165 
Battery,  thermo-electric,  109 

voltaic,  definition  of,  103 

voltaic,  plunge,  104 
Bichromate  cell,  96 
Box  form  of  bridge.  68 
Brackets  for  incandescent  elecj 

trie  lamps,  16 !,  167 
Bridge,  box  form  of,  68 

electric,  65 

arms  of,  65 

operation  of,  65 

origin  of  term,  66 


Bridge,  sliding  form  of,  67 

use  of,  in  determining  electric 

resistance,  65,  66.  67 
Brush-and-spray  discharge,  232 
Brushes,  collecting,  297 

distributing,       of        electric 

motor,  332 
Bunsen's  voltaic  cell,  97 


r^\  &  C.  electric  motor,  340 
^    Calorimeter,  electric,  meth- 
od for  measurement  of  cur- 
rent strength,  11 
Candle,  Jablochkoflf,  154,  155 
Capillary  electrometer,  50 
Carbon  arc,    exhibition   of,   by 

Davy,  145 

Carbon  collecting  brushes,  297 
electrodes,  artificial,  158 
electrodes,     cored    for    arc 

lamps,  160 

electrodes  of  arc  lamps,  man- 
ufacture of,  158 
electrodes,  copper  coating  of, 

159 

electrodes,   various   position 
of,  in  voltaic-arc  lamp,  148 
holders  for  a»%c  lamps,  118,  149 
Carbonizable    fibrous    material, 
use  of,  in  incandescent  elec- 
tiic  lamp,  164 
Carbon  voltaic  arc,    appearance 

of,  147, 148 
Cell,  Bunsen's  voltaic,  97 
dry  vokaic,  103 
galvanic,  88 
photo-electric,  114 
selenium,  114 
selenium.  Van  Uljanin's,  117, 

118 
standard,  Clark's  H-form  of, 

51.52 
standard  voltaic,  Clark's,  51 


INDEX. 


417 


Cell,  standard  voltaic,  definition 

of,  51 
standard   voltaic,    Fleming's 

form  of,  53 
standard     voltaic,     Lodge's 

form  of,  54 
standard  voltaic.  Rayleigh's 

modification  of  Clark'sform 

of.  52 
roltaic,  88 

voltaic,  bichromate,  96 
voltaic,   chemical  avoidance 

of  polarization  of.  93 
voltaic,  chemical  phenomena 

in,  88,  89 

voltaic,  closed  circuited.  102 
voltaic,    counter -electromo- 
tive force  of,  93 
voltaic,    Daniell's,  constant, 


voltaic,   DanielPs,   cause  of 

constancy  of,  99 
voltaic,  double-fluid,  9* 
voltaic,     elecirical   phenom- 
ena in,  89 

voltaic,  electro-chemical 
avoidance  of  polarization 
of,  93 

voltaic,  gravity,  100 
voltaic,  Grove's  nitric  acid, 

97 

voltaic,  Leclanche,  101 
voltaic,    mechanical    avoid- 
ance of  polarization  of,  93 
voltaic,  open-circuited.  102 
voltaic,  polarization  of,  93 
voltaic,  poles  of,  90,  91 
voltaic,  single  fluid,  91 
voltaic.  Smee.  95 
voltaic,  source  of  energy  of, 

89 

Cellulose,  u<»e  of  for  incandescing 
filament,  of  electric  lamps, 
174,  175 


Chamber  of  incandescent    elec- 
tric lamp,  164 

cause  of  decrease  of   trans- 
parency of,  179, 180 
exhaustion  of,  171,  172, 173 
Choking  coil,  221 

method  of  operation  of,  221 
Circuit,  constant-current,    defl 

nition  of,  134 
constant-potential,  135 
constant-potential,  definition 

of,  131 

deflection  of,  by  magnet,  317 
series  distribution,  134 
Circle  of  reference,  190 
Circuits.priinary  and  secondary, 

of  transformers,  212 
Circumstances  influencing  elec- 
tric resistance,  81 
Clamond's  thermo-electric  pile, 

H3 
Clark's  form  of  standard  voltaic 

cell,  51 
H-forn,  of  jtaadard  voltaic 

cell,  51 
Classification  of  dynamo-electric 

machines,  299,  300 
Closed-circuited  transformers, 279 

voltaic  cell.  102 
Clutching  devices  for  lamp-rod 

of  arc  lamp,  149 
Coil,  choking,  221 

choking,  method  of  operation 

of,  221 

induction,  269 
Henry's  induction,  276,  277 
kicking,  221 
reaction,  221 

Rhumkorff  induction,  273 
RhumkorfF  induction,  action 

of,  275 

Coils,  inverted  induction,  272 
proportionate,  69 
resistance,  construction  of,  63 


418 


INDEX. 


Coils,  resistance,  methods  of 
winding,  63 

series,  of  compound-wound 
machines,  135 

shunt,    of    compound-wound 

machines,  135 
Collecting  brushes,  297 

of  dynamo-electric  machine, 

289 
Commutator,  action  of,  299 

method  of  action  of,  293, 
291 

of  dynamo-electric  machine, 
288 

segments,  cause  of  wearing 
of,  294 

two-part,  295 

Complex -harmonic  motion,  190 
Compound     winding    for    auto- 
matic regulation,  135 
Condensers,  use  of,  in  connection 
with    alternating    current 
distribution,  220 

Conductor,  incandescing,  for 
electric  lamp,  properties 
requisite  for,  163 

modern  conception  of  the 
manner  of  the  passage  of  a 
current  through,  194 

effects  of  temperature  on  re- 
sistance of,  80 

Constant-potential  distribution 
system  for  alternating  cur- 
rents, 208 

current  circuit,  definition  of, 
134 

potential  circuit,  definition 
of,  134 

current,  definition  of,  125 

current  dynamo-electric  ma- 
chines, 129,  130, 131 

current  system  of  e  ectrical 
distribution,  127 

potential  circuit,  135 


Constant  potential  dynamo-elec- 
tric machines,  135 
Continuous  current,    definition 

of,  125 
rotation  produced  by  electric 

current,  323 
Cored  carbon  electrodes   for  arc 

lamps,  160 
Counter-electromotive    force  of 

motor,  effect  of,  338 
of  voltaic  cell.  93 
Couple,  voltaic,  SS 

voltaic,  polarity  of,  90 
thermo-electric,  107 
Crater  at  positive  carbon  elec- 
trode of  voltaic-arc  lamp,146 
Crossover     trolley     telpherage 

system,  358,  359 
Crystal,  pyro  electric,  118,  119 
Current,  alternating,  definition 

of,  125 

alternating,  density  of,  pass- 
ing through  a  conductor, 
192,  193 

constant,  definition  of,  125. 
continuous,  definition  of,  125 
direct,  definition  of,  125 
distribution,  alternating,  207 
Currents,  alternating,  185 

electric,   methods  for   meas- 
urement of,  7 
sinuous,  322 
Curve  of  electromotive  forces  of 

alternating  currents,  1»6 
Cut-out,  film,   for   incandescent 

lamps,  177 
safety,  138 

safety,  for  incandescent  elec- 
tric lamp,  177 

DANIELL'S   constant  voltaic 
cell,  98,  99 

Davy,  exhibition  of  carbon  vol- 
taic arc  by,  145 


INDEX. 


419 


Dead-beat  action  of  galvanom- 
eter, 28 

galvanometer,  27 
Deprez-D'Arsonval      galvanom 

eter,  27 

Detector  galvanometer.  30 
Devices,  automatic  safety,   for 
series-connected  arc  lamps, 
151 

electro-receptive,'    multiple- 
connected,  133 

electro-receptive,  series-con- 
nected, 128 

Dielectrics,  effects  of  tempera- 
ture on  resistance  of,  81 
surrounding  conductors,  part 
played  by  in  propagation 
of  current  through  said  con- 
ductors. 194 

Differential  galvanometer,  26 
Dimmer,  222 

construction  and  connections 

of,  222. 223 
Direct    current     definition    of, 

125 
Directed-streaming     discharge, 

242 
Discharge,  brush-and-spray,  232, 

233 

coil,  Tesla's  disruptive,  241 
directed-streaming,  242 
flaming.  231 

luminous-disc-shaped,  243 
of  Leyden  jar,  alternating  or 
oscillatory  character  of,  198 
rotating-brush,  244,  245,  246 
sensitive-thread,  230 
high-frequency,  227 
moderately     high     and    ex- 
cessively high-frequencies, 
differences     between,    227, 
228 

high-frequency,    effect      of, 
on  human  body,  229 


Disjunctor,  uses  of,  220 
Disruptive  discharge  coil,  Tes- 
la's, 241 
Distributing  brushes  of  electric 

motor,  lead  of,  332,  333 
Distribution  of  alternating  cur- 
rents by  constant  potential, 
208 

of  electricity  by   alternating 
currents,    advantages    of, 
209,  210 
of  electricity  by   direct    or 

continuous  currents,  125 
of  electricity,  classification  of 

different  systems  for.  126 
of  electricity,  three-wire  sys- 
tem of,  138,  139,  140 
series,  127,  128 

system  for  alternating  cur- 
rents, enumeration  of  parts 
required  for,  207 
Double-carbon    arc    lamp,    153, 

154 

Double-fluid  voltaic  cell,  94 
Drum-armature,  290 
Dry  voltaic  cell,  103 
Dynamo  electric  machine,  287 
alternating  current.  216,  217 
alternating,  synchronism  of, 

187, 188 
and  electric  motor,  analogies 

between,  329 
armature  of,  288 
building  up  of,  304 
collecting  brushes  of,  289 
commutator  of,  288 
field  magnets  of,  288 
parts  entering  into  the  con- 
struction of,  288 
pole  pieces  of,  288 
reaction  principle  of,  304 
reversibility  of,  328 
series,  301,  302 
shunt,  303 


420 


INDEX. 


Dynamo-electric  machines,  classi 
flcation  of,  299,  300 

series  and-separately-excited, 
305 

ehunt-and  separately-excited 
306 

shunt-and  series-wound,  307 

varieties  of,  301 

Dynamos  of  various  types,  direc- 
tion of  rotation  of,  when 
traversed  by  a  current, 


TT^DISON-HOWELL  lamp    in- 

J-^    dicator,  141 

Edison's    pyro-electro  magnetic 

motor,  342,  343 
Eel,  electric,  1*1, 122 
Effect,    screening,    of  metallic 
plates  to   inductive  influ- 
ence   of    alternating    dis- 
charges, 199 
Electric  balance,  65 

bombardment  lamps,  237,  238, 

239 

bridge,  65 
lamp,  series,    incandescing, 

176, 177 

lamp,  Tesla's  incandescent- 
ball,  239 
meter,  33 
motor,  327 

motor,  efficiency  of,  330 
motor,  reversal   of   rotation 

of,  333,  334 
railroads,   overhead    system 

for.  363 

time  meter,  34 

Electric  railroads,  surface  system 
for,  363 

system  of  operation  of,  361 
underground  system  for,  363 
Electric  transmission  of  power, 
351 


advantages  possessed    by, 

352,  353,  354 
Electrical  phenomena  in  voltaic 

cell,  89 

Electricity    and  heat,  flow    of, 
resemblances  between,  195 
Electricity,  distribution  of,    by 
direct  or   continuous  cur- 
rents, 125 

description  of  system,  126 
multiple-arc  distribution  of, 

133 

parallel  distribution  of,  133 
pyro-,  1 18,  119 
series  distribution  of,  132 
system  of    constant  current 

distribution,  127 
system     of    multiple-arc    or 
parallel  distribution  of,  127 
thermo ,  Siebeck's  discovery 

of,  107 
three-wire  system  for  the  dis 

tribution  of,  138, 139, 140 
Electro-chemical  meter,  33 
dynamic  induction,  253,  251 
dynamic  induction,  varieties 

of.  255,  256 
dynamics,  311 
dynamics,  definition  of,  211 
dynamometer,  Siemen's,  31,32 
magnetic  induction,  cause  of, 

261,  262,  263 

magnetic   induction,    defini- 
tion of,  255 
magnetic  meter,  33 
Electromotive     force,     methods 

for  measurement  of,  39 
Electro-receptive  devices,    mul- 
tiple connected,  133 
series  connected,  128 
Electro  statics,  definition  of,  311 
Electro-thermal  meter,  34 
Electrodes  for  arc  lamps,  copper 
coating  of,  159 


INDEX. 


421 


Electroliers,  166 
Electrolyte,  definition  of,  88,  89 
Electrolytes,  effects  of  tempera- 
ture on  resistance  of.  81 
Electrometer,  attracted  disc,  47 

capillary,  50 

capillary,  phenomena  of,  120 

needle  of.  45 

quadrant,  14 

sectors  of,  15 
Electrometer- voltmeter,  44 

definition  of,  40 
Element,  thermo-electric,  107 
Energy,  source  of,  in  voltaic  cell. 

Exhaustion  of  incandescent  elec- 
tric lamp  chamber,  171, 172, 
173 

Eye,  selenium,  116, 117 


FEEDING     devices      of     arc 
lamps,  149 

Field  magnets  of    dynamo-elec- 
tric machines.  288 
Filament,  incandescing,  164 

of  electric  lamp,  carbonizing 

material  for,  168 
of   electric    lamp,     flashing 

process  for,  169  170 
of    electric    lamp,   occluded- 

gas  process  for,  169. 170 
of  electric  lamp,  preparation 

of,  167 
of  electric  lamp,  process  for 

forming  joint  at  junction 

with  leading-in  wires,  170, 

171 
incandescing,      of      electric 

lamp,  shaping  of  material 

for,  166,  167 
incandescing  of  electric  lamp, 

varieties    of    forms  given 

to,  174 


Filament  of  incandescent  electric 
lamp,  method  of  mounting, 
171 

Film  cut-out  for  incandescent 
electric  lamps,  177 

Flaming  discharge,  231 

Flashing  process  for  carbon  fila- 
ment of  incandescing  elec- 
tric lamp,  169, 170 

Fleeming  Jenkin's  telpherage 
system,  356 

Fleming's  rule,  264,  265 
standard  voltaic  cell,  53 

Flow  of  heat  and  electricity,  re- 
semblances between,  195 

Frog,  galvanoscopic,  86 

Fuse,  safety,  137 

for  multiple  connected  incan- 
descent electric  lamp,  178, 
179 

C\  ALVANIS  experiment,  85 
^*    Galvanic  cell,  88 
Galvano     rnetr.c     uiethod     for 
measurement    of    electro- 
motive force,  39 
Galvanometer,  astatic,  17 

ballistic,  3J 

dead  beat,  27 

dead  beat,  action  of.  28 

Deprez  D'Arsonval,  27 

detector,  30 

magnetic  screen  for,  25 

differential,  25 

marine,  '25 

mirror,  21 

principles  underlying  the  op- 
eration of,  13 

reflecting,  21 

sensibility  of,  20 

sine,  23 

Schweigger's  invention  of,  12 

tangent,  24 

torsion,  28 


INDEX. 


Galvanometer,  vertical,  29 

voltmeter,  construction  of,  41 
voltmeter,  definition  of,  40 
voltmeter,  Sir  William  Thom- 
son's, 42 

voltmeter,  various  methods 
for  deflection  of  needle  of, 
41 

Galvanometers,  64 
commercial,  20 
Galvanoscopic  frog,  86 
Generator,    pyro  magnetic,    280, 

281,282 
German-silver    wire,   use  of,  in 

resistance  coils,  64 
Gravity  voltaic  cell,  100 
Grove's  nitric  acid  voltaic  cell,  97 
Gauge,  round-wire,  76 
wire-and  plate,  79 
vernier-wire,  77,  78 
Guard  plate  of  electrometer,  47 
wire  shade  for  incandescent 
electric  lamp,  180 

Tq  AND  regulation,  136 
•*-*-       regulator,  137 
Heat    and    electricity,    resem- 
blance, between  flow  of,  195 
Henry's  induction  coil,  276,  277 
High    frequency   discharge  ap- 
paratus, Thomson's,  247 
discharge,  Tesla's  system  of 

lighting,  238,  239 
discharges,  227 
Hood  and  outrigger,  for  arc  lamp 

157 

of  arc  lamp,  156 
Hot  St.  Elmo's  fire,  235,  236 

IMPEDANCE,  definition  of ,  192 
formula  for,  192,  193 
geometrical     representation 

of,  192 
nature  of,  191 


Incandescent-ball  electric  lamp, 

239 
Incandescent  electric  lamp,  life 

of,  179 

chamber  of,  164 
hermeoical    sealing    of,    173, 

174 

parts  of,  161,  165 
pendants  for,  166 
porcelain  shade  for,  180 
reflectors  for,  180 
socket  of,  165 

straight-filament,  Tesla's,  240 
Swan's,  174, 175 
switches  for,  166 
use  of    alternating  currents 

in,  218,  219 

wire  shade-guard  for,  180 
lamps,  brackets  for,  166, 167 
lighting,  163 
lamp,  164 

Incandescing  filament,  164 
Independent  system   of  electric 

railroads,  361 
Indicator,  lamp.  Edison-Howell, 

141 

Inductance,  definition  of,  255 
Induction  coil,  269 

coil,  definition  of,  269 
direction     of    current     pro- 
duced in,  270 

manner  of  action   of,  269,  270 
or  transformer,  varieties  of, 

272 
dynamo  electric,     definition 

of,  256 

electro-dynamic,  253 
how  produced,  254 
electromagnetic,   cause    of, 


electro-magnetic,     definition 

of,  255 

in  armature  loop,  298 
magneto-electric,  cause  of,261 


INDEX. 


Induction  magneto-electric,  defl 

nition  of,  255 

mutual,  cause  of,  256,  257,  260 
mutual,  definition  of,  255 
produced  by  alternating  cur 

rent,  effect  of,  191 
self,  cause  of,  256,  267 
self,  definition  of,  255 
voltaic  current,       definition 

of,  255 

Inverted  induction  coils.  272 
Iron-clad  transformers,  279 

TABLOCHKOFF    candle,    151, 
W     155 
Jacoby's  law  of  maximum  effect. 

339 
Jar,  porous,  of  voltaic  cell,  97 

REY  switches  for  incandes- 
cent electric  lamp,  166 
Kicking  coil,  221 

LAMP,  ARC.  voltaic,  position 
of  carbons  in,  118 
electric  incandescent,  carbon 
izing    material    for   incan 
descing  filament,  168 
electric,  flashing  process  for 
incandescing    filament  of, 


electric,  occluded-gas  process 

for  incandescing  filament, 

169, 170 
electric,  preparation  of  incan 

descing     filament   for.    167 
electric,  process  for  forming 

joint  at  junction  of  leading- 

in   wire   and  incandescing 

filament,  170,  171 
electric,  shaping  of  material 

for    incandescing  filament 

of,  167 


incandescent  electric,  parts 
of,  154,  165 

indicator,  Edison  Howell,  141 
Lamps,  arc,  various  construc- 
tions of,  US,  149 

electric   bombardment,    237, 


Lvw  of  maximum  effect,   Jaco 

by's,  339 
of  Poynting,  196 

Laws  of  electro  dynamics,  Am- 
pere's, 311,312 

Lead  of  distributing   brushes  of 
electric  motor,  332,  333 

Leadmg-in  wires  for  incandes- 
cent electric  lamps,  164, 
165 

Leclanche  voltaic  cell,  101 

Leyden  jar,  alternating  or  oscil- 
latory character  of  the  dis- 
charge of,  198 

Life  of  incandescent  electric 
lamp,  179 

Light  emitting  power  of  voltaic 
arc,  effect  of  temperature 
on,  146 

Lighting,  incandescent  elec- 
tric, 133 

Live  trolley  crossing,  365 

Lodge's  form  of  standard  voltaic 
cell,  54 

Luminous  disc  shaped  discharge, 
243 

MACHIXE,   dynamo-electric, 
287 

parts  entering  into  the  con- 
struction of.  288 

Machines,  dynamo -electric/  con- 
stant-current, 129, 130,  131 
Machines,  dynamo-electric,  con- 
stant-potential, 1~5 
Magnet,  deflection  of  circuit  by, 
317 


424 


INDEX. 


Magnetic   fields,  mutual   action 

of,  319 

needle,  methods  for  remem- 
bering direction  of  deflec 
tion  of,  14 

screens  for  watche?,  20D 
Magnetization,  anomalous,  198 
Magneto  -  electric       induction, 

cause  of,  261 

induction,  definition  of,  25j 
Manufacture     of     carbon    clec 

trodes  for  arc  lamps,  158 
Marine  galvanometer,  25 
Mercurial  air  pump,  Sprengel's, 

171,  172 

Meter,  electric,  33 
electric  time,  34 
electro-chemical,  33 
electro-magnetic,  33 
electro-thermal,  34 
Motion,  complex-harmonic,  190 
simple-harmonic  or  periodic, 

illustration  of,  189,  190 
simple-periodic  or  harmonic, 

definition  of,  18 
Motor,  electric,  327 

electric,  C.  &  C.,  340 
electric,  circumstances  of  its 
successful          competition 
•with  steam.  355,  356 
electric,     counter-electromo- 
tive force  produc  d  by,  337, 
338 

-generators,  219 
Sprague's  electric,  340 
electric,  distributing  brushes 

of,  332 

Multiple-arc  distribution  of  elec- 
tricity. 133 

or  parallel  system  for  the 
distribution  of  electricity 
127 

Multiple  connected       electro-re- 
ceptive devices,  133 


Multiplier.  Schweigger  s,    15,16 
Mutual  action  of   parallel   cur- 
rents. 316 
Mutual  induction,  definition  of, 

255 
cause  of,  256.  .'57,  260 

"VTEEDLE,  astatic,  18 

-*-^     astatic,  deflecting  action  of 

current  on,  19 
of  electrometer.  45 

Negative  carbon  electrode  of  vol- 
taic arc  lamp,  nipple  at,  146 
plate  of  voltaic  cell,  91 
pole  of  voltaic  cell,  90,  91 

Neutral  wire  of  three  wire  sys- 
tem of  distribution,  140 

Nipple  at  negative  carbon  elec 
trode  of  voltaic  arc  lamp, 
146 

Nobilli's  thermo-electric  pile  110, 
111 

Non-conductors,  effects  of  tem- 
porature  on  resistance  of. 


/  \CCLUDED-3  AS  process  for 
^    incandescent    filament    of 

electric  1  imp,  172,  173 
Oersted,  discovery  of,  12 
Ohm,  standard,  73 
Ohmmeter,  Ayrton's,  74,  75 
Ohmmeters,  20 
Open  circuited  transformers,  279 

voltaic  cell,  102 

Outrigger  and  hood  for  arc  lamp, 
157 

for  arc  lamp,  157 

PAIR,  voltaic,  88 
Parallel  circuits,  action  of, 
on  one  another,  320 
parallel  connection  of  alter- 
nators, 187 


INDEX. 


425 


Parallel  currents,  action  of,  on 

each  other,  316 
distribution  of  electricity,  133 
Path,  alternative,  phenomena  of, 

193,  197 

Pendants  for  incandescent  elec- 
tric lamps,  166 

Period  of  alte  matin?  current,  186 
Phase  of  alternating  current,  186 
Phenomena,  chemical,  in  voltaic 

cell,  88,  89 

of  alternative  path,  193, 197 
thermo  electric,  107 
Photo-electric  alarm,  116 

cell,  114 

Photophone,  116 

Physiological  action  of  high  fre- 
quency   discharges     upon 
the  human  body,  2-J8,  229 
Pile,  thermo  electric,  110 
voltaic,  87,  88  , 

Plants,  effect  of  electricity  on, 

121 

Plate  and  wire  gauge,  79 
Plate,  positive,  of  volt  »ic  cell,  91 
Platinum,  use  of,  in   leading-in 
wires  for  incandescent  elec- 
tric lamps,  170, 171 
Platinoid,  use  of,  in  resistance 

coils,  64 

Plugs,  safely,  137 
Plug-keys  for  resistance  coils,  63 
Plunge  battery,  104 
Polarity  of  voltaic  couple,  90 
Polarization  of  voltaic  cell,  93 
chemical  avoidance  of,  93 
electro  chemical     avoidance 

of,  93 

mechanical  avoidance  of,  93 
Pole,  analogous,  119 
antilogous,  119 
armature,  292 

negative,  of  voltaic  cell,  90, 
91 


Pole-piece    of     dynamo-electric 

me  chine,  288 

positive,  of  voltaic  cell,  93.  91 
Poles  of  voltaic  cell.  90,  91 
Porous  jar  of  voltaic  call.  97 
Porte-electric  system,  359.  360 
Positive  carbon  electrode  of  vol- 
taic arc  lamp,  crater  at,  146 
plate  of  voltaic  cell,  91 
pole  of  voltaic  cell,  90,  91 
Potentiometer,  48 
Power,  electric  transmission  of, 

351 

Poynting's  law,  196 
Process,  flashing,  for  carbon  fila- 
ment of  incandescent  elec 
trie  lamp,  169,  170 
Proportionate  coi's.  68 
Pyro  electric  crystal,  118,  119 
Pyro-ma?metic  motor,  Edison's, 

342,  343 

Pyro-electricity,  118,  119 
Pyro  magnetic      generator,    280, 


/QUADRANT  electrometer,  44 


"DAILROADS,      electric,     de- 
-*-*'    pendent  system,  definition 

of,  359 
electric,  independent  system, 

definition  of,  359 
trolley  wheel  for,  3C3 
Rayleigh's        modification       of 

Clark's    form   of  standard 

voltaic  cell,  52 
Reaction  coil,  221 

coil,  balanced,  223 
Reckenzaun's  reversing  gear  for 

electric  motors,  334.  335 
Rectangular  equivalent  of  sinu- 

ous current,  322 
Reflecting  galvanometer,  21 


426 


INDEX. 


Reflectors  for  incandescent  elec- 
tric lamps,  180 
Regulation,    automatic,    Brush 

system  of,  131 

automatic,  for  constant  cur- 
rent, 129 

automatic,     Thomson-Hous- 
ton system  of,  130 
hand,  136 

Regulator,  hand,  137 
Repulsion  between  angular  cur- 
rents, 316 
Resistance  coils,  construction  of, 

63 
coils,  effects  of  temperature 

on,  80 

electric,  circumstances  influ- 
encing, 81 

Resistance  electric,  determina- 
tion of  by  substitution 
method,  62 

electric,  measurement  of,  61 
electric,  method  of  determin- 
ing by  comparison  with  gal 
vanome'.er  deflections,  64 
electric,    methods    of  deter- 
mining, 61 

electric,    methods    of   deter- 
mining, by  differential  gal- 
vanometers, 64 
telenium,  115 

Reversing  gear  for  electric  mo- 
tors, Reckenzaun's,  334,335 
Resistance,  spurious,  192 
Reversibility  of  dynamo-electric 

machine,  328,  329 
Rheostat,  Wheatstone'a,  74 
Rhumkorff  induction  coil,  273 
induction    coil,    action     of, 

275 

Ring  armature,  291 
Rotating-brush     discharge,    244, 

245,  246 
Rule,  Fleming's,  264,  265 


SAFETY  cut  out,  138 
cut-out    for  incandescent 

electric  lamp,  177 
fuse,  137 

fuse  for  multiple-connected, 
incandescent  electric  lamp, 
178 

plugs,  137 
strips,  137 
Saturated  solution,  definition  of 

56 

Schweigger's  multiplier,  15,  16 
Screen,   magnetic,    for    galvan- 
ometer, 25 
Screens,  magnetic,  for  watches, 

200 

Sectors  of  electrometer,  45 
Segments  of  commutator,  cause 

of  wearing  of,  294 
Selenium  cell,  114 
resistance,  115 
Self-induction,  cause  of,  256,  257 

definition  of,  255 
Sensitive-thread  discharge,  230 
Semi-inc^ndesent  electric  lamp, 

160 

Series-and  separately-excited  dy- 
namo-electric machines,  305, 
306 

and    shunt-wound    dynamo- 
electric  machines,  307' 
coils    of     compound-wound 

machines,  135 

connected  arc    lamps,  auto- 
matic safety  devices  for,  151 
connected     electro-receptive 

devices,  128 

connection     of     alternating 
currents  impracticable,  187 
distribution,  127,  128 
distribution  circuit,  134 
dynamo-electric  machine,  301, 

302 
distribution  of  electricity,  132 


INDEX. 


427 


Series-incandescent  electric  lamp, 

176, 177 

thermo-electric,  108 
Shades,  porcelain,  for  incandes- 
cent electric  lamp,  180 
Shunt-and  separately-excited  dy- 
namo-electric machines,  306 
coils    of    compound    wound 

machines  135 

dynamo  electric  machine,  303 
magnet,  operation  of,  in  arc 

lamps,  150 
Siebeck's  discovery  of   thermo 

electricity,  107 

Siemens'  discovery  of  th«  re- 
action principle  of  the 
dynamo-electric  machine* 
304 

electro-dynamometer,  31,  32 
Simple-harmonic  motion  defini- 
tion of,  188 

Simple  periodic  motion,  defini- 
tion of,  188 

Simple-periodic  or  harmonic  mo- 
tion, illustration  of,  189,  190 
Sine,  galvanometer,  23 
Single-fluid  electric  cell,  94 

voltaic  cell,  94 
Sinuous  current,  322 

rectangular  equivalent  of,  322 
Sliding  form  of  electric  bridge, 

67,  70,  71,  72 
Smee's  voltaic  cell,  95 
Socket  of  incandescent  electric 

lamp,  165 

Solenoid,  magnetic  attraction 
and  repulsion  exerted  by 
318 

magnetic     properties      pos- 
sessed by,  318 
Solution,    saturated,    definition 

of,  56 
supersaturated,  definition  of, 


Soren  Hjorth's  discovery  of  the 
reaction  principle  of  the 
dynamo-eleclric  machine. 
304 

Sparks,  T-shaped,  248.  249 
three  branched,  248 
Y-shaped,  218,  249 

Sprague  electric  motor,  341 

Sprengel's  mercurial  air  pump, 
171.172 

Spurious  resistance,  192 

St.  Elmo's  fire,  hot,  235,  236 

Standard  ohm.  73 

Step-up  transformer,  definition 
of,  278 

Step-down  transformer,  defini- 
tion of,  278 

Storage  system,  359,  358 

Straight-filament  incandescent 
electric  lamp,  210 

Streaming  discharge,  232 

Substitution  method  for  deter- 
mining electric  resistance, 
62 

Strips,  safety,  137 

Supersaturated  solution,  defini- 
tion ol,  56 

Swan's  incandescent  electric 
lamp,  174,  175 

Switch,  automatic  photo-electric, 
116 

Synchronism  of  alternating 
currents,  187,  188 

System  for  electric  transmission 
of  power,  essential  parts  of, 
349 
of   multiplearc    or    parallel 

distribution  of,  127 
storage,  357,  358 

rr-SHAPED  sparks,  248,  249 

*  Tangent  galvanometer,  24 
Temperature ,  effects  of,  on  resist- 
tance  coils,  80 


428 


INDEX. 


Tesla's     disruptive     discharge 
coil,  241 

high-frequency       discharge, 

system  of  lighting,  238.  239 

incandescent  -  ball     electric 

lamp,  239 

straight- filament     incandes- 
cent electric  lamp,  210 
Thermo  electric  battery,  109 
combination,  107 
couple,  107 
element,  107 
phenomena,  107 
pile,  110 

pile,  Clamond's.  113 
pile,  Nobili's,  111.  110 
series,  108 

Thermo-electricity,  Siebeck's  dis- 
covery of,  107 

Thomson's    high-frequency    dis- 
charge apparatus,  217 
Three-branched  sparks,  248 
Three- wire  system  for  (he   dis- 
tribution of  electric!  Ly,  133, 
139, 140 
Torque,  definition  of,  330 

relation    between    load  and, 

330 

Torsion  galvanometer,  28 
Transformer,  269 
circuits,  211 

ratio  of  transformation,  271 
step-up  and  step  down,  defi- 
nition of,  278 

automatic  regulation  of,  213 
cause  of   loss   of   energy  in, 

279.  280 

closed  circuited,  279 
definition  of,  27  > 
iron-clad,  2:9 

musical  note  emitted  by,  218 
open- circuited,  279 
primary  and  secondary  cir- 
cuits of,  212 


Transformer,  use  of,  in  direct  or 
continuous  current   distri- 
bution of  electricity,  219 
use  of,  in  systems  of  alter- 
nating    current    distribu- 
tion. 211 
varieties  of,  272 
Trolley  cross-over,  363 

pole,  363 

Two-part  commutator,  action  of 
armiture     coils     on    seg- 
ments of,  296 
commutator,  295 

UNSTEADINESS        of      arc 
lamp,  causes  for,  159 

TTAN    ULJANIN'S  selenium 


\ 


cell,  117,  118 


Vernier  wire  gauge,  77,  78 
Vertical  galvanometer,  29 
Volta'.s  investigations,  85 
Voltaic  arc,    exhibition    of,   by 

Davy,  145 
Voltaic  battery,  definition  of,  103 

cell,  88 

couple,  88 

current  induction,  definition 
of,  255 

pair,  88 

pile,  87,  88 

Voltameters,  classification  of,  8 
Voltameter,  sulphuric  acid,  9 

volume,  9 

weight,  10 

Voltametric    method  for  meas- 
urement   of    electric  cur- 
rent, description  of,  8 
Voltmeters,  20 
Voltmeter,  definition  of.  40 

IT  ATTMETER,  20 
*      construction  of,  44 
definition  of,  43 


INDEX. 


429 


Wheathtone's  discovery  of  the 
reaction  principle  of  the 
dynamo-electric  machine, 
304 

electric  balance,  65 
electric  bridge,  65 
rheostat,  74 
Wheel,  Barlow 's,328 
Wire  gauge,  round,  76 
Wire  and  plate  gauge,  79 
gauge,  vernier,  77,  78 


neutral,  of  three-wire  system 

of  distribution,  140  • 
Wires,  leading-in,  of  incandes- 
cent electric  lamp,  161,  165 


Y 


-SHAPED  sparks,  218,  249 


C  plate   of    voltaic    cell, 
amalgamation  of,  92 


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that  date  the  author  has  been  led  to  prepare  an  entirely 
new  series  of  primers,  but  of  a  more  advanced  charac- 
ter in  consonance  with  the  advanced  general  knowl- 
edge of  electricity. 

Electricians  will  find  these  primers  of  marked  inter- 
est from  their  lucid  explanations  of  principles,  and  the 
general  public  will  in  them  find  an  easily  read  and 
agreeable  introduction  to  a  fascinating  subject. 

CONTENTS. 

I.— Effects  of  Electric  Charge.  II.— Insulators  and 
Conductors.  III. — Effects  of  an  Electric  Discharge. 
IV. — Electric  Sources.  V. — Electro-receptive  Devices. 
VI— Electric  Current.  VII.— Electric  Units.  VIII. 
— Electric  Work  and  Power.  IX. — Varieties  of  Elec~ 
trie  Circuits.  X. — Magnetism.  XI. — Magnetic  Induc- 
tion. XII.— Theories  of  Magnetism.  XIII.— Phenom- 
ena of  the  Earth's  Magnetism.  XIV,— Electro-Mag- 
nets. XV-— Electrostatic  Induction.  XVI.— Frictional 
and  Influence  Machines.  XVII.— Atmospheric  Elec- 
tricity. XVIII.— Voltaic  Cells'.  XIX.— Review,  Prim, 
er  of  Primers. 

PUBLISHED  AND  FOR   SALE  BY 

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THE 

Electrical  Transmission  of  Intelligence 

.  ftrjd  Other  flduanced  Primers  of  Electricity 

BY  PROF,  EDWIN  J,  HOUSTON,  A.M., 

Author  of 
"4  Dictionary  of  Electrical  Words,  Terms  and  Phrases,* 

&C.,  &C.,  &6. 


CLOTH.   PRICE:,  $1.00. 


The  third  and  concluding  volume  of  Prof.  Houston's 
Series  of  Advanced  Primers  of  Electricity  is  devoted  to 
the  telegraph,  telephone,  and  miscellaneous  applica- 
tions of  the  electric  current. 

In  this  volume  the  difficult  subjects  of  multiple  and 
cable  telegraphy  and  electrolysis,  as  well  as  the  tele- 
phone, storage  battery,  etc.,  are  treated  in  a  manner 
that  enables  the  beginner  to  easily  grasp  the  principles, 
and  yet  with  no  sacrifice  in  completeness  of  presenta- 
tion. 


I.  The  Eleetric  Transmission  of  Intelligence.  II. 
The  Electric  Telegraph.  III.  Multiple  Telegraphy. 
IV.  Cable  Telegraphy.  V.  Electric  Annunciators  and 
Alarms.  VI.  Time  Telegraphy.  VII.  The  Telephone. 
VIII.  Electrolysis.  IX.  Electro-Metallurgy.  X.  Stor- 
age or  Secondary  Batteries.  XI.  Electricity  in  War- 
fare; Electric  Welding.  XII.  Some  Other  Applications 
oi  Electricity.  XIII.  Klectro-Therapeutics.  XIV. 
lie  view,  Primers  of  Primers. 

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The  Electric  Railway 

IN  THEORY  AND  PRACTICE. 

By  O.  C.  CROSBY  and  Dr.  LOUIS  BELL. 

Second  Edition.    Revised  and  Enlarged. 

•416  Octavo  Pages,  182  Illus.    Price,  $2. BO. 

This   is    the  first    SYSTEMATIC     TREATISE  that  has 

been  /,„/<//>/,,</  on  the  ELECTRIC  RAIL  WAY,  and 

it  is  intended  la  cover  the  ti E \KRAL  VR1 JV- 

CIPLES  Of  DESK1N,  CO  \STRUC- 

T/OJV  Afflt  OPERATION. 


Chapter    I.     General  Electrical  Theory. 
Prime  Movers. 
Motors  and  Car  Equipment. 
The  Line. 

Track,  Car  Houses,  Snow  Machines. 
The  Station. 

The  Efficiency  of  Electric  Traction. 
Storage  Battery  Traction. 

L     Miscellaneous  Methods  of  Electric  Traction. 
High  Speed  Service. 
Commercial  Considerations. 
Historical  Notes. 


Appendix  A.     Electric  Railway  vs.  Telephone  Decisions. 

B.  Instructions  to  Linemen. 

C.  Engineer's  Log  Book. 

"  D.     Classification  of   Expenditures  of   Electric   Street 

Railways. 

E.  Concerning      Lightning     Protection,      by      Prof. 

Elihu  Thomson. 

F.  Motors  with   Beveled   Gear,   and     Series   Multiple 

Control  of  Motors. 

"  G.     Method    of   Measuring  Insulation     Resistance    of 

Overhead  Lines, 


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RECENT  PROGRESS 

IN 

ELECTRIC    RAILWAYS 

BEING  A  SUMMARY  OF  CURRENT  PROGRESS 

IN  ELECTRIC)  RAILWAY  CONSTRUCTION, 

OPERATION.  SYSTEMS,  MACHINERY, 

APPLIANCES,  &c.,  COMPILED 

By   CARL    HERINC. 

886  pages  and  120  illustrations.    Cloth,         -        Price,  $1.00 


This  volume  contains  a  classified  summary  of  the 
recent  literature  on  this  active  and  promising  branch 
of  electrical  progress,  with  descriptions  of  new  appa- 
ratus and  devices  of  general  interest. 


CONTENTS. 

Chapter  I. — Historical.  Chapter  II. — Development 
and  Statistics.  Chapter  III. — Construction  and  Opera- 
tion. Chapter  IV. — Cost  of  Construction  and  Opera- 
tion. Chapter  V.— Overhead  Wire  Surface  Roads. 
Chapter  VI. — Conduit  and  Surface  Conductor  Roads. 
Chapter  VII.— Storage  Battery  Roads.  Chapter  VIII. 
—Underground  Tunnel  Roads.  Chapter.  IX.  -High 
Speed  Interurban  Railroads.  Chapter  X.— Miscellan- 
eous Systems.  Chapter  XI.— Generators,  Motors  and 
Trucks.  Chapter  XII.— Accessories. 

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AN    IMPORTANT   NEW   BOOK. 


ALTERNATING   CURRENTS 

AN 

ANALYTICAL  AND  GRAPHICAL  TREATMENT 
FOR  STUDENTS  AND  .ENGINEERS. 


BY 

BEDELiIi,   Ph.D.,   and 
,  Ph.D.,  (Cornell  Univ.) 


Uniform  with  "  the  Electric  Railway"  by  Crosby  and  Bt4. 


Cloth.    325  pages  and  112  Illustrations.     Price,  $2.60. 


While  there  are  many  monographs  and  special  treatises  on 
alternating  currents,  they  are  either  fragmentary  or  special  in 
character,  or  couched  in  mathematical  language  requiring  a 
special  mathematical  education  to  interpret. 

In  this  volume  the  theory  of  alternating  currents  Is,  for  the 
first  time,  treated  in  a  connected  and  logical  manner,  and  in 
mathematical  language  familiar  to  the  ordinary  mathematical 
public,  while  the  graphical  extension  can  be  followed  by  those 
not  having  a  special  knowledge  of  mathematics. 

Some  parts  of  this  volume  have  been  published  in  separate 
papers,  and  from  the  cordial  welcome  they  received,  it  is  be- 
lieved that  the  present  work  will  fill  a  distinct  want  In  an  im- 
portant branch  of  electrical  science. 


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EXPERIMENTS   WITH 

ALTERNATE  CURRENTS 

Of  High  Potential  and  High  Frequency, 

II. >    NIKOLA   Tlvsi,  \. 


156  pages,  witJi  Portrait  and  35  Illustrations. 
Cloth,  $1.00. 


This  book  gives  in  full  Mr.  Tesla's  important  lecture 
before  the  London  Institution  of  Electrical  Engineers, 
which  embodies  the  results  of  years  of  patient  study 
and  investigation  on  Mr.  Tesla's  part  of  the  phenomena 
of  Alternating  Currents  of  Enormously  High  Fre- 
quency and  Electromotive  Force. 

EVERY  ELECTRICIAN,  ELECTRICAL  ENGINEER  OR 
STUDENT  OF  ELECTRICAL  PHENOMENA  WHO  MAKES 
ANY  PRETENSIONS  TO  THOROUGH  ACQUAINTANCE 
WITH  RECENT  PROGRESS  IN  THIS  IMPORTANT  FIELD 
OP  RESEARCH  WHICH  MR.  TESLA  HAS  SO  ABLY  DE- 
VELOPED MUST  READ  AND  REREAD  THIS  LECTURE. 

The  book  is  well  illustrated  with  35  cuts  of  Mr. 
Tesla's  experimental  apparatus,  and  contains  in  ad- 
dition a  biographical  sketch,  accompanied  by  a  full- 
page  portrait,  which  forms  a  fitting  frontispiece  to  a 
lecture  which  created  such  widespread  interest. 


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Alternating  Currents  of  Electricity: 

Their  Generation ,   Measurement,  Distributioij 
and  Application. 


By  Gisbert  Eapp,  M.I.C.E.,  M.I.E.E. 

«  ii  li    an  Introduction  'i>>    William  Stanley,  Jr. 


Cloth.     164  Pages,  37  Illus.,  2  Plates.     $1.00. 

This  volume  explains  in  clear,  simple  language  the 
theory  of  alternating  currents  and  apparatus,  ^particular 
attention  being  paid  to  transformers  and  multi-phase 
currents  and  motors. 

The  treatment  is  entirely  a  practical  one,  the  descrip- 
tions noting  the  various  advantages  and  defects  of  dif- 
ferent types,  and  the  sections  devoted  to  designing 
containing  the  practical  data  and  instructions' required 
by  the  engineer. 

CO3STTE3STTS. 

Introduction,  by  William  Stanley,  Jr.  Chap.  I.  In- 
troductory Chap.  II.  Measurement  of  Pressure,  Cur- 
rent and  Power.  Chap.  III.  Conditions  of  Maximum 
Power.  Chap.  IV.  Alternating  Current  Machines. 
Chap.  V.  Mechanical  Construction  of  Alternators. 
Chap.  VI.  Description  of  Some  Alternators.  Chap. 
VII.  Transformers.  Chap.  VIII.  Central  Stations  and 
Distribution  of  Power.  Chap.  IX.  Examples  of  Cen- 
tral Stations.  Chap.  X.  Parallel  Coupling  of  Alterna- 
tors. Chap.  XI.  Alternating  Current  Motors.  Chap. 
XII.  Self-Starting  Motors.  Chap.  XIII.  Multi-phase 
Currents. 

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PRINCIPLES    OR 

DYNAMO  ELECTRIC  MACHINES 

AND 

Practical  Directions  for  Designing 
and  Constructing  Dynamos, 

By  CARL    HERINO. 

Sixth  Thousand.    279  pages.     5!)  illustrations         Price,  $2. SO. 


CONTENTS, 

Review  of  Electrical  Units  and  Fundamental  Laws. 

Fundamental  Principles  of  Dynamos  and  Motors. 

Magnetism  and  Electromagnetic  Induction. 

Generation  of  Electromotive  Force  in  Dynamos. 

Armatures. 

Calculation  of  Armatures. 

Field  Magnet  Frames. 

Field  Magnet  Coils. 

Regulation  of  Machines. 

Examining  Machines. 

Practical* Deductions  from  the  Franklin  Institute  Tests 

of  Dynamos. 

The  So-called  "Dead  Wire"  on  Gramme  Armatures. 
Explorations  of  Magnetic  Fields  Surrounding  Dynamos. 
Systems  of  Cylinder-Armature  Windings. 
Table  of  Equivalents  of  Units  of  Measurements. 


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ORIGINAL   PAPERS 

ON 

DYNAMO  MACHINERY 

AND    ALLIED    SUBJECTS. 

By  JOHN  HOPKINSON,  F.R.S. 

Uniform  with  Thompson's  "  Lectures  on  the  Electromagnet." 
fRICK,  INCLUDING  POSTAGE,     $1.OO. 


This  collection  of  papers  includes  all  written  on 
electro- technical  subjects  by  the  distinguished  author, 
most  of  which  have  been  epochal  in  their  character 
and  results. 

The  papers  are  arranged  according  to  subject.  Five 
papers  relate  wholly  or  in  part  to  the  continuous  cur- 
rent dynamo  ;  four  are  on  converters  and  one  each  on 
the  theory  of  alternating  current  machines  and  on  the 
application  of  electricity  to  light-houses. 

In  the  words  of  the  author  "The  motive  of  this 
publication  has  been  that  I  have  understood  that  one 
or  two  of  these  papers  are  out  of  print  and  not  so  acces- 
sible to  American  readers  as  an  author  who  very  greatly 
values  the  good  opinion  of  American  electrical  engi- 
neers would  desire." 

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THE  ELECTRIC  MOTOR 

AND  ITS  APPLICATIONS. 


By  T.  C.  Martin  and  Joseph  Wetzler,  with  nn 

appendix  bringing  the  book 'down  to 

date  by  Dr.  Louis  Bell. 


325  Large  Quarto  Pages,  and 
354  Illusirations.    PRICE, 
pontage  prepaid  to  any 
part  of  the  World. 


This  timely  work  is  the  first  American  Book  on 
Electric  Motors,  and  the  only  book  in  any  lan- 
guage dealing  exclusively  and  fully  with  the 
modern  Electric  Motor  in  all  its  various  practical 
applications.  The  book  is  a  handsome  quarto, 
the  page  being  of  the  same  size  as  Dredge's  large 
work  on  "Electric  Illumination,"  and  many  of 
the  culs  are  full  page. 

No  effort  has  been  spared  to  make  the  book 
complete  to  date,  and  it  wiil  prove  invaluable  to 
every  one  interested  in  the  progress  and  develop- 
ment of  the  Electric  Motor  or  the  Electrical 
Transmission  of  Energy 

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The 

Electromagnet, 

BY 

Prof.  SILVANU3  P.  THOMPSON,  D.S3.,  B.A.,  1LLE.E. 

A  full  theoretical  and  practical  account  of  the  proper- 
ties and  peculiarities  of  electromagnets,  together 
with  complete    instructions  for  designing 
magnets  to  serve  any  specific  purpose. 
Published  with  the  express  con- 
scot  and  careful  revision  of 
the  author. 

Cloth.    280  Pages.    75  Illustrations. 
Price,  $1.00. 

LECTORS  I.:  Introductory;  Historical  Sketch:  Goner-lities 
Concerning  Elec' romagnets ;  Typical  Forms;  Polarity;  Uses 
in  General;  The  Properties  of  Iron;  Methods  of  Measuring 
Permeability;  Traction  Methods;  Curves  of  Magnetization 
and  Permeability  ;  The  Law  of  the  Electromagnet ;  Hysteresis . 
Fallaciesand  Facts  about  Electromagnets.  LECTURE  II.:  Gen- 
eral Principles  ol  Design  and  Construction ;  Principle  ot  tti3 
Magnetic  Circuit,  LECTURE  HI.:  Special  Designs;  Winding  of 
theCopp  r;  Windings  for  Constant  Pressure  and  for  Constant 
Current ;  Miscellaneous  Rules  about  Winding ;  Specifications 
for  Electromagnets  ;  Amateur  Rules  about  Resistance  of  lilcc- 
tromagnet  and  Battery  ;  Forms  of  Electromagnets ;  I  ffect  of 
Size  of  Coils;  Effect  of  Position  of  Coils;  Effect  of  Shape  of 
Section ;  Effect  of  Distance  between  Poles ;  Researches  of 
Prof.  Hughes;  Position  and  Form  of  Armature;  Pol  -Pieces 
on  Horseshoe  Magnets.  Contrast  between  Electromagnets  and 
Permanent  Magnets:  Electromagnets  for  Maximum  1  victim; 
Electromagnets  for  Maximum  Ranee  of  Attraction  ;  Electro- 
magnets of  Minimum  Weight:  A  Useful  Guiding  Principle; 
Electromagnets  for  Use  with  Alternating  Currents;  Eleciro- 
magncts  for  Quickest  Action ;  Connecting  Coils  for  Quickest 
Action;  Battery  Grouping  for  Quickest  Action  :  Short  Cores 
TS.  Long  Cores.  LECTURE  IV.:  Electromagnetism,  etc. 

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ELECTRIC  LIGHTING 

SPECIFICATIONS 

FOR   THE  USE   OF 

ENGINEERS  AND  ARCHITECTS, 

By   E.  A.  MERRILL. 

The  author  has  drawn  up  a  set  of  specifications  covering  the 
various  classes  of  lighting  Installations,  which  will  serve  as 
forms  for  any  special  type  or  character  of  plant,  and  which  are 
at  the  same  time  full  enough  to  cover  the  ordinary  Installation 
of  electrical  apparatus  and  electric  light  wiring.  The  book  will 
prove  especially  useful  to  architects  and  engineers  who  desire  a 
full  knowledge  of  the  necessary  requirements  of  the  various 
classes  of  electrical  installations  la  order  to  meet  the  demands 
of  the  insurance  inspectors  and  the  conditions  of  safety.  The 
latest  rules  are  given  of  the  (l)  National  Electric  Light  Asso- 
ciation.  (2)  National  Board  of  Fire  Underwriters.  (3)  New 
England  Insurance  Exchange. 

OTHER    CONTENTS: 

Specifications  for  the  Installation  of  Electric  Lighting  Plants. 
—General  Specifications.— Installation  of  Dynamos  isnd  Switch- 
boards.—Alternate  Current  converter  System.  Constant  Poten- 
tial.— General  Specifications  for  Alternate  or  Direct  Current  Dy- 
namos for  Parallel  system  of  Distribution.— Arc  Dynamos.— Fix- 
tures, etc.— Interior  \Viring.-T\vo  Wire,  Diivct  or  Alternating 
Current  System.— Three-Wire  System.—  Three- Wire  System 
Adapted  to  Two-wire  System.— Arc  System.— Conduit  System, 
Two-Wire.— Int erior  Wiring  lor  Central  station  Plants.— Pole 
Lines. — Low  Potential,  Direct  Current,  Two  or  Three- Wire. -^ 
Alternating  System.— Street  Lighting  Circuits.— Specifications 
for  Steam  Plant. 

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STANDARD  TABLES 


ELECTRIC  WIREMEN 


WITH    INSTRUCTIONS    FOR    WIREMEN    AND    LINEMEN, 

RULES  FOR  SAFE  WIRING,  DIAGRAMS  OF 

CIRCUITS  AND  USEFUL  DATA. 

By  CHAS.    M.    DAVIS. 

Third  Edition,  Thoroughly  Revised  and  Edited  by 
W.  D.  WEAVER, 

Clotb,  -  -  -  Price,  $1.OO. 


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form basis  and  arranged  in  a  more  convenient  manner 
for  practical  use. 

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thoroughly  reliable  and  practical  in  its  data  and  free 
from  verbiage  and  padding. 


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OP 


STATIC   ELECTRICITY, 

•WITH  FULL  DESCRIPTION  OF  THE  HOLTZ  AND  ToPLKR 
MACHINES  AND  THEIR  MODE  OF  OPERATIOH. 

Bj   PHILIP    ATKINSON,   A.JL,    Ph.».> 

Cloth,  12mo;  228  Pages;  64  Illustrations. 


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The  author  of  this  treatise  has  made  a  special  study 
of  Static  Electricity,  and  is  an  acknowledged  master 
of  the  subject.  The  book  embodies  the  result  of. 
much  original  investigation  and  experiment,  which. 
Dr.  Atkinson's  long  experience  as  a  teacher  enables 
him  to  describe  in  clear  and  interesting  language, 
devoid  of  technicalities. 

The  principles  of  electricity  are  presented  Untram- 
mclcd,  as  far  as  possible,  by  mathematical  formula), 
so  as  to  meet  the  requirements  of  a  large  class  who 
have  not  the  time  or  opportunity  to  master  the  in- 
tricacies of  formulae,  which  are  usually  so  perplexing 
to  all  but  expert  mathematicians. 

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many  years'  experience  in  the  class  room,  the  lecture 
room  and  the  laboratory,  and  were  adopted  only- 
after  the  most  rigid  test  of  actual  and  oft-repeated 
experiment  by  the  author. 

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PRACTICAL  INFORMATION 
TELEPHONISTS. 

BY  T.  D.  LOCKWOOD, 

Electrician  American  Bell  Telephone  Company. 


12MO,  192  PAGES;  CLOTH. 
PRICE,  81.00 

CONTENTS. 

Historical  Sketch  of  Electricity  from  600  B.  O.  to  1882  A.  D. 
Facts  and  Figures  about  the  Speaking  Telephone. 
How  to  Build  a  Short  Telegraph  or  Telephone  Line. 
The  Earth  and  Its  Relation  to  Telephonic  Systems  of  Com 

munication. 
The  Magneto-Telephone— What  it  is,  Uow  it  is  Made,  and 

How  it  Should  be  Handled. 
The  Biake  Transmitter. 

Disturbances  Experienced  on  Telephone  Lines. 
The  Telephone  Switch-Board. 
A  Chronological  Sketch  of  the  Magneto-Bell,  and  How  to 

Become  Acquainted  with  it. 
Telephone  Transmitter  Batteries. 
Lightning— Its  Action  upon  Telephone  Apparatus— How  tc 

Prevent  or  Reduce  Troubles  Arising  Therefrom. 
The  Telephone  Inspector. 
The  Telephone  Inspector— His  Daily  Work. 
The  Inspector  on  Detective  Duty. 
The  Daily  Routine  of  the  Telephone  Inspector. 
Individual  Calls  for  Telephone  Lines. 
Telephone  Wires  versus  Electric  Light  Wires. 
Electric  Bell  Construction,  Part  I. 
Electric  Bell  Construction,  Part  II. 
Housetop  Lines,  Pole  Lines  and  Aerial  Cables. 
Anticipations  of  Great  Discoveries  and  luveutions. 


the  above  book  will  be  sent  by  mail,  POSTAGE 
PREPAID,  to  any  address  in,  the  world,  on  receipt  of  price, 
Addresf 

THE  W.  J.  JOHNSTON  CO.,  Ld., 

Times  Building,  New  York. 


THE  PIONEER  ELECTRICAL  JOURNAL  OF  AMERICA. 


Read  wherever  ttie  English  Lanpage  is  spoken, 


THE  ELECTRICAL  WORLD 

is  tlie  large*!,  most  handsomely  Illustrated,  and 

most  widely  circulated  electrical  journal 

in  i In-  world. 

It  should  be  read  not  only  by  every  ambitious  elec- 
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The  paper  is  ably  edited  and  noted  for  explaining 
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discoveries  in  simple  and  easy  language,  devoid  of 
technicalities.  It  also  gives  promptly  the  most  com- 
plete news  from  all  parts  of  the  world,  relating  to  the 
different  applications  of  electricity. 


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UNIVERSITY  OF  CALIFORNIA  AT  LOS  ANGELES 

THE  UNIVERSITY  LIBRARY 
This  book  is  DUE  on  the  last  date  stamped  below 


1 

jSJ  14 


QC 

523   Houston  - 
HSle — Electrical — 
measurements 


AUfi  8 


QC 
523 

H81e 


